harmonic noise 1400 rpms

Jackie Morris · #1 03-09-2005, 06:25 AM

my motor makes a strange harmonic noise around 1400 -1700 rpms when pulling normally. it's not a knocking noise but some kind of rhythmic type sound. 1995 K1500 5.7 211,000 miles, any ideas?

J-Ri · #2 03-09-2005, 07:07 AM

Is it like a pulsating sound going between high and low frequencies? That's most likelt sympathetic vibrations between your engine and anything else with a similar natural frequency. Replacing the harmonic balancer on the front of the crankshaft may help (the rubber starts to separate when they get old), but it really isn't hurting anything.

Jackie Morris · #3 03-09-2005, 08:27 AM

Quote:

Originally Posted by J-Ri

Is it like a pulsating sound going between high and low frequencies? That's most likelt sympathetic vibrations between your engine and anything else with a similar natural frequency. Replacing the harmonic balancer on the front of the crankshaft may help (the rubber starts to separate when they get old), but it really isn't hurting anything.

It's a weird kind of nosie, it would almost make you think it's the cam shaft vibrating in the bearings. I'll look at the harmonic balancer and see how the rubber looks.

Wheeler88 · #4 03-09-2005, 09:33 AM

Is it a little more noticable when going around a high speed curve? I'm thinking front wheel bearings.

Jackie Morris · #5 03-09-2005, 12:23 PM

Quote:

Originally Posted by Wheeler88

Is it a little more noticable when going around a high speed curve? I'm thinking front wheel bearings.

it's not a wheel bearing, you can control it with the throttle, you can speed up the motor and get above the rpm and it stops or below it and it doesn't do it. it's a strange sounds and you can really hear it pulling a hill real slow about 1400-1700 rpms.

gocougs · #6 03-09-2005, 01:10 PM

Unfortunately I don't have an answer, but I have a very similar, if not the same problem with my 98, K1500, 99,000 miles. Mine occurs between 1200 and 1500 RPM. It started about 50,000 miles ago. It only occurs under load and is not vehicle speed dependent, only engine RPM.

I don't alway hear it but I can always feel it in the gas pedal, which is where I first noticed it. Since it feel it in the gas pedal, I figured it must be something on the firewall resonating, but haven't had any luck yet finding it. I've pulled the heat shields but that wasn't it. It drives me nuts, so I try to avoid the rpm all together, which isn't always easy.

bfug · #7 03-09-2005, 02:20 PM

You might want to look at your carrier bearing. While this may sound odd when it starts to go it will send noise and vibrations through the entire body making it very hard to find. The way I thought mine was a brake or wheel bearing dragging. When I jacked up my truck and put it on jack stands. I ran it in gear and found it after I got under the truck. Good Luck

J-Ri · #8 03-09-2005, 04:19 PM

The K series doesn't have a carrier bearing. The driveshaft is short enough to not need one due to the transfer case taking up some space.

Does it do it at that speed reguardless of where the gas pedal is?

Jackie Morris · #9 03-09-2005, 04:34 PM

Quote:

Originally Posted by J-Ri

The K series doesn't have a carrier bearing. The driveshaft is short enough to not need one due to the transfer case taking up some space.

Does it do it at that speed reguardless of where the gas pedal is?

it doesn't matter where the gas pedal is just the rpm's, once
your are above or below 1400-1700 it doesn't do it.

J-Ri · #10 03-09-2005, 05:14 PM

Timing perhaps? Are you using quality fuel?

acewelderman · #11 03-09-2005, 06:22 PM

Sounds like a u joint complaining. The harmonic balancer is a suspect to.

bfug · #12 03-09-2005, 07:27 PM

Sorry folks,
My bad not knowing that the K series did not have a carrier bearing.

J-Ri · #13 03-10-2005, 11:20 PM

If it was a U joint, wouldn't it be there when the vehicle speed was whatever, not engine RPM?

Don't be sorry about not knowing that, there's nothing wrong with trying to help.

kenny-1907 · #14 03-11-2005, 12:10 PM

Hey jackie, do you happen to have an after market muffler or dual exhaust on your truck? After reading this i paid attention to mine on the way home the other day and mine makes the same noise as did my 96 sierra and my 88 K1500 jimmy. All of which have had dual exhaust and that is the hum i can hear from mine at approx the same rpm as you described. Just a thought.

Jackie Morris · #15 03-11-2005, 03:21 PM

Quote:

Originally Posted by kenny-1907

Hey jackie, do you happen to have an after market muffler or dual exhaust on your truck? After reading this i paid attention to mine on the way home the other day and mine makes the same noise as did my 96 sierra and my 88 K1500 jimmy. All of which have had dual exhaust and that is the hum i can hear from mine at approx the same rpm as you described. Just a thought.

yep, I'm running a V-Force muffler with dual outlet for the tail pipes, no cat. Been talking to some mechanics and one of the thinks maybe the thrust bearing on the crankshaft. with 211, 000 miles on it he could be right but oil pressure is still over 45 psi while driving.

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