The Stovebolt Site Forums The Shop Area Driveline 72 blazer 4wd with 350 and auto transmission

Trying to ID the source of a klunck coming from "under" area just in front of the 4 wheel drive stick/shifter on transmission hump. Basically under the floor heater vent. Truck runs, shifts, acellerates fine. Sound, a klunck, occurs at any speed and whether or not under load, when I lift my foot off the acellerator. Does not affect anything. Running in 2 high with hubs free.

U joint? Any input appreciated. Thanks.

Is this a random noise or does it have a rythem to it?

A bad needle bearing in the U-joint will make a snapping sound when they start binding and breaking into small pieces. Jack the truck up and put the trans in neutral and see if there is any play in the u-joints.

Can you feel it in the shift levers?

Could it be a motor mount coming off of it's seat and then klunking back in place?

Grabbing straws here...

Mike B

Thanks for the suggestions mike. It is a single klunck when I let off gas. I can be going 15 mph on flat land and if I take foot off accellerator one klunck as I remove foot. No more noises or at any other time. Can't feel it otherwise.

Trans./engine mounts will transmit noises many places. Check all mounts.

Ed

Thanks Ed. Will do.

Front driveshaft on those will wear. A lot were "all time" from the factory. Some had hubs added later so get under it and give that driveshaft a good wiggle. If it is loose get it fixed. They can cause a lot of trouble($$$Money).

Thanks Ed. Will do.

Just to be clear, it does not klunk when not moving. Wanted to be sure it's not the accelerator linkage arm since you said "when I lift my foot". I assume it does not do it when just sitting in neutral?....or motor off?...and lift foot? Loose linkage would hit firewall just about where you said noise is. Could also be broken motor, tranny or Xfer case mount. I guess when you say klunk you mean heavy duty sound not linkage hitting? A shift of something heavy? U-joints are almost impossible to check by hand unless really, really bad. You have to take driveshaft off. Don't think the front drive shaft would make noise in 2WD.

Ed

The noise only occurs when driving. It does not occur when I let up on the gas, a little or punch it and let up. It only happens when I let up all the way (basically remove my foot). Most noticeable (probably due to road noise) when going about 35 and I take my foot off the accellerator. One moderate thunk at that exact time every time. It doesn't affect drivability, but I want to fix if it will. My list is:

Check motor, tranny and xfer case mounts
Check driveline wiggle

I don't think it's the linkage. It's a heavier noise than that.

Thanks so much for your input.

Thanks Steve

Drop the rear end cover plate and inspect the carrier housing for an oblong hole where the spider gear pivot shaft goes through it. It's a chronic problem area on rear ends from those years, and if the transmission has ever had a shift kit installed it's much more likely to happen.
Jerry

Had that happen on the front ring gear carrier on my low mileage 79 GMC 4x4 many years ago.