Low whining when going in reverse... transmission?

I have an '01 Maxima with about 110K miles on it. In the last few weeks, it started making a low whining noise when going in reverse. My first impulse was the brakes. I change them myself, but the local tire shop will check them for free, so I had them take a look and they said the rear breaks are fine till at least next spring. So after finding out that it's not the brakes, I started listening more carefully and realized that it is making that noise the entire time I'm moving in reverse, not just when my foot is on the brake.

I talked to a friend of mine who thinks the transmission could be slipping. Any other ideas? Second opinions? Anyone agree with the transmission theory? He told me one thing I could do to test it would be to put it in reverse, hold my foot on the brake and put a little gas on. If it just rev's up with good torque, then I'm ok, if it jams and shakes, then it's the tranny. Any other tests I can do?

I've had plans to get the transmission flushed for quite some time now, so I think this weekend I will get that done and have them check this problem out for me at the same time. But I'd like to go in with a little knowledge, so I don't get raped at the mechanic shop. Transmissions to me are like big magic black holes. I know nothing about them at all. Any ideas would be appreciated. Thanks!

Update: I just did as my friend told me and revved the engine a bit in reverse while I had my foot on the brake. I couldn't really hear any actual grinding or banging, but there was a bit of squeaking or creaking, which I assumed was frame structure stretching or something like that. Maybe that was the sound I was supposed to be hearing, I don't know.

Further Update: It's not the transmission! Ok, I just did some more testing. (i was at work when I posted this, so now that I'm home, I'm doing some fidling.) I jacked up the back of the car, and took the ebrake off so the rear wheels could spin freely. The drivers side wheel spun fine with no problems or sounds. However the passenger side wheel, while still spinning fairly freely, whined like a banshee. And it is a metalic whinining like something is rubbing against the rotor. But when the tire shop checked the brakes they said they were fine. So what the heck is rubbing on the rotor? When I get time I'll take the wheel off and do some closer inspection.

... However the passenger side wheel, while still spinning fairly freely, whined like a banshee. And it is a metalic whinining like something is rubbing against the rotor. But when the tire shop checked the brakes they said they were fine. So what the heck is rubbing on the rotor? When I get time I'll take the wheel off and do some closer inspection.

Small stone, rusty bits from rotor found its way between pads. And/or handbrake wire too tight and/or jamming, keeping the brake on just a little bit...

Open and clean the caliper assy - or just drive and enjoy. (Note: Rear caliper piston goes in ONLY by rotating)

Small stone, rusty bits from rotor found its way between pads. And/or handbrake wire too tight and/or jamming, keeping the brake on just a little bit...

Open and clean the caliper assy - or just drive and enjoy. (Note: Rear caliper piston goes in ONLY by rotating)
You're probably right. I need to just tear the sucker open and find the culprit. But what do you mean that the piston goes in only by rotating? Not sure I follow you. Edit: Oh maybe I understand. As the brakes wear down, the piston will go out on it's own, but to put new brakes on, (which of course I'm going to do if I'm ripping the calipers off) I need to turn the piston to get it to go back in. Now am I following you?

Thanks for the reply.

You're probably right. I need to just tear the sucker open and find the culprit. But what do you mean that the piston goes in only by rotating? Not sure I follow you. Edit: Oh maybe I understand. As the brakes wear down, the piston will go out on it's own, but to put new brakes on, (which of course I'm going to do if I'm ripping the calipers off) I need to turn the piston to get it to go back in. Now am I following you?

Thanks for the reply.

Yes. Inside the rear caliper is a 'jacking' screw which extends out when pads wear and piston travels further out... this screw is to counter hand brake wire return pull spring; piston wont return by itself in zero position. They even sell a 10$ tool for turning the piston (big screwdriver is enough; used sideways).

New =thicker pads wont fit in - if piston is not pushed/rotated fully in.

Cant believe the noise is serious b/c that shop did take a peek in... any dirt may make nasty noise. But if the noise is bad, new pads seldom hurt :)

Front calipers - no screw - as no handbrake - just push in (SAAB with front wheel hb is an exception)

Also lube the pistons ONLY with special brake grease; check http://www.cardomain.com/ride/748507/18
...and get 10% better braking pwr.

Yes. Inside the rear caliper is a 'jacking' screw which extends out when pads wear and piston travels further out... this screw is to counter hand brake wire return pull spring; piston wont return by itself in zero position. They even sell a 10$ tool for turning the piston (big screwdriver is enough; used sideways).

New =thicker pads wont fit in - if piston is not pushed/rotated fully in.

Cant believe the noise is serious b/c that shop did take a peek in... any dirt may make nasty noise. But if the noise is bad, new pads seldom hurt :)

Front calipers - no screw - as no handbrake - just push in (SAAB with front wheel hb is an exception)

Also lube the pistons ONLY with special brake grease; check http://www.cardomain.com/ride/748507/18
...and get 10% better braking pwr.
Well the noise is REALLY loud, very noticable. It's loud in the cabin, and blaring outside. The reason the shop didn't hear it is because they never drove the car around in reverse. Just pulled it through the shop. And I'm sure all they did was take the tires off and look at the pads. They didn't take the brake assembly apart.

... They didn't take the brake assembly apart.

The caliper structure today is pretty failsafe (not anymore like my 'weekly failing' -43 jeep brakes), thus a quick peek will surely show if any safety hazard exist. Minuscle dirt particles can still surely make terriblese noises...

New pads will push piston in, also a deterrend against piston rust (with fluid swap).

So I just took it in and had it checked out. They said that it is the internal pad that has worn down, and it is just the indicator making the noise. (don't ask me why they didn't catch that last week when I had them check the brakes). Anyway, what would cause the inner pad to wear quicker than the outer pad?

Update: I went ahead and changed the pads today. That piston was a bit of a bitch to twist back in. Still not sure why they wore unevenly like that, maybe it's normal for the inner pad to wear quicker, I don't know. One thing I do know is that these were the original pads, never been changed, so it's a nice feeling to have to nice new pads on there. And the sound is gone!

... Still not sure why they wore unevenly ...

Rails need lube - or need even replacement. If rusted, buy new...

The caliper assy consist of two, double-rail connected parts moving smoothly against each other. They should slide easily... u didnt check the link = return to the issue: wheel out and special lube into the three spots.

Now read this - ONLY SPECIAL lube - or brakes will catastrophically fail.

Rear calipers for that make/model have a reputation for sticking. Problem in design or whatever causes the caliper piston to stick, not nessesarily the slide rails. Nap is right stick rails would cause uneven wear though. I've had to replace both rears on my wife's 2001 SE, oh and the drivers side stuck so hard it heated up the backing plate so hot it wiped out ABS wheel speed sensor & cost me another $150 for the part and $100 for $tealer$hip to diagnose. (no one but Nissan can read ABS code on 2000 & above)

Rear calipers for that make/model have a reputation for sticking. Problem in design or whatever causes the caliper piston to stick, not nessesarily the slide rails. Nap is right stick rails would cause uneven wear though. I've had to replace both rears on my wife's 2001 SE, oh and the drivers side stuck so hard it heated up the backing plate so hot it wiped out ABS wheel speed sensor & cost me another $150 for the part and $100 for $tealer$hip to diagnose. (no one but Nissan can read ABS code on 2000 & above)
Yeah, I noticed that the slide rails actually moved quite nicely, in fact in trying to pull the caliper assembly off, it kept sliding around on the rails, so I suspect that it probably is a sticky caliper. I'm sure in fact. Because trying to slide that sucker back in, even with the tool I rented from AutoZone, was quite hard. I had to spray some brake cleaner fluid all over to losen it up and keep the rubber boot from twisting around. But it's all done now, and if the previous brakes went this long, I'm sure I'll be rid of this car before I need to change those rear brakes again.

... But it's all done now, and if the previous brakes went this long, I'm sure I'll be rid of this car before I need to change those rear brakes again.

Automotive brake fluid is hydroscopic; meaning that is absorbs moisture over time: even the closed system 'sucks' in water and system rusts inside out. This is why fluid has to be swapped once in 2-3 yrs; good lube helps also in this issue. (This is typical in 'any' vehicle.)

Another issue is how brakes are used: sunday style driving seldom activates rear brake pistons ... time to time made full pedal push (pistons move) is a bonus.