rattle in OD and when it drops to 4th on hills

[Solomon7]

I've had my 96 4liter H.O. AW4 NP231 Cherokee about 7months now I just changed tires to 30 BFG and have stock 3.55 gearing . I go up the same hill pretty often and after the tire swap i have to drop to 4th on long hills now . Well she starts at 60 ish at +/-2000 rpm's (guages) and in about a quarter mile the OD gets bogged down speed drops steadily to about 45 and a rattle (metallic) comes in faintly from I think near the tranny. I pump it and it drops in 4th or 3rd at aroun 3000 rpms and the rattl is abit worse so I let off the throttle and ride it out at 40+mph like a wuss. What the heck is that rattle and how do I make it stop!?
BTW 124000 on the clock

[AlohaBra]

It either ping or a cracked exhaust header, I think.

[Solomon7]

ping?? Sorry I'm a bit of a noob . I've heard that term before but if you could define that would rock. There is a bit of a knock at idle, but an old Jeeper said that's just the way a Jeep's idle sounds, like their trademark. Bull$*#t or not???

[Solomon7]

Anyone? ping definition? got new plugs wires cap and rotor>

[AlohaBra]

Ping is a ticking noise when you get on it hard up a hill.

It is due to weak ass octane gas or a lean mixture or just too much tire for the gearing.

Put some mid grade gas in it and if it goes away...it is ping.

[Solomon7]

Thanks. I'll try going to higher octane this weekend up the hill and see if that does it. It is more of a metallic rattle kind of "blender on a tin can" sound. That's a ping? I'm just worried about the tranny. Fluid looks like crap, but am changing today>

[fredjacksonsan]

Rattle could be an exhaust shield that's moving around with the lower rpm's. Check to see if anything underneath is loose.