Clunking noise when accelerating hard in LOW

[Orangecrush]

Here's one that maybe someone has encountered and can comment on- When I put my truck in LOW (either 4 or 2) and accelerate, I get a "clunk, clunk, clunk, clunk, clunk" coming from somewhere underneath. It sort of sounds like a tire hitting some part of the body, but I don't think that's it. My old Corvette had a similar problem where the differential inpute yoke would hit the body while turning because the whole assembly would shift upward under heavy load. I don't think that's the case here though either.

Just throwing it out there in case it's something real common that I don't know about. I hope it's nothing serious, just some item that has too much play and is hitting something else, while turning, under load..?

 

[S_bolt19]

Check your front CV joint at the transfer case. What is happening is that when you engage the "LO", whether you lock the hubs or not, is that the front driveshaft starts to turn. The clunking that you are hearing is the CV joint that is shot. And how do I know this? I had the same problem 3 years ago. Pretty simple fix, take it to a driveline shop and have them put a new one on or have a new driveshaft built. If you have a new shaft built, I would HIGHLY recommend Tom Wood Custom Driveshafts. They are the best around. It has been a while since I had one built from them, but, they are worth every penny you spend on them.

 

[Seabronc]

If it sounds like a ****** of Leprechaun beating on your truck with a Shalale. It could also be caused by weak springs doing what is called spring wrap which caused the drive shaft to hit the underneath of the truck.

Good luck,

>-

 

[Orangecrush]

Yep, first thing I thought of was that the drive shaft was hitting the underside (as this is what happened with the corvette....the sound is almost the exact same too). However, I got under the truck and can find no place where the drive shaft would hit, nor can I find any marks from it hitting anywhere. I'll look again.

As for the cv joint explanation, you may be right. It does do this in low 2 or low 4, and regardless of if I lock the hubs or not. HOWEVER- the "clunk, clunk, clunk" noise is deeper than the typical cv joint noise coming from the front of a front-wheel drive import. I usually think of a cv joint problem to sound like "click, click, click" not "clunk......". Just clarifying the sound that I'm getting. It's really just about the same noise that I would get if I turn the wheel all the way in one direction and the knobs from my 35"s catch the side of the body "clunk, clunk, clunk". Also, it only does it under load. If I just cruise, there's no noise. If I hog on it, I get the noise. This is what caused me to assume something is being rocked out of place due to the torque and thus hitting something. Would CV's do this under load ONLY?

The rear drive shaft is new. the front is old. Oh well, one more thing to repair.

 

[STLKIKN]

I had a sound that seems similar in one of my 77s... turned out the tranny mounts were weak and under hard acceleration (only under hard accel) it would clunk as you describe.

It was hard to find, but the rear shaft would just touch the body when the case wound up...

 

[S_bolt19]

I am gonna stick with the CV joint. Reason is that if it were a rear driveshaft, you would get that noise every time you drove, not just in LO gears. Crawl under it and grab the upper joint on the front shaft and you will know immediately if it is your problem. Any type of movement (as little as 1/16") will make a BIG noise. And yeah, it will be a "clunk, clunk" not a "click, click". That, by the way is a great way to describe /emoticons/[email protected] 2x" width="20" height="20" /> .