98 GMC Sierra

[deadman1474]

Yes in my frends pickup while he was pulling a 20 foot boat you here what sounds like a midget with a hammer under the car. Like a metal on metal banging so we pulled over called a dodge ram to come get the boat. So with no load we took off everything seemed fine untill we hit over 2000 rps. Then same noise and almost like a loss of power. It seems kindof like the trans but ive never heard a trans knock before. Any suggestions?

[DSW]

Sounds like the driveshaft or U joint might be out of balance. Is there a clunk as you put it in drive or reverse? Thats usually a good sign that a U joint may be going. You can also look at the drive shat and see if it lost a weight.

My suburban made similar noises when the rear U joint took a cr@p. The banging didn't start until I got it up to speed.

[STwelder]

Don't sound like a U joint to me. It sounds like a rod knocking, but I'd have to hear it my self to be sure.
My experience with U joints are that they tend to screech with each revolution even barely moving less than 5mph. For your sake, hope it's the U joint.

[deadman1474]

Don't sound like a U joint to me. It sounds like a rod knocking, but I'd have to hear it my self to be sure.
My experience with U joints are that they tend to screech with each revolution even barely moving less than 5mph. For your sake, hope it's the U joint.

It dosent do it when you just rev it up only when its in gear tho.

[Joe H]

Sounds like a rod to me too. Although I have had a very similar experience with an engine and all that was wrong was the knock sensor on the enginne had become disconnected. Without the sensor the timing advanced far enough to cause some severe detonation. It would knock like crazy anytime you loaded the engine.

[daddy]

I had a 98 Tahoe that made some strange tapping noises from underneath. It was inconsistent in rythm and seemed to come and go randomly. It turned out to be in the transfer case. I finally got tired of it and yanked the case and split it. I found the main bearing on the output shaft had shredded its cage, and the balls were just kind of collecting in the bottom of the race and occasionally making their way around... hence the random tapping. I could not believe that it did not come apart. I replaced the bearing, and all was well. It was the case with the auto 4wd option that requires the special blue juice that you get only from GM. The blue fluid solved the slip/stick, chirping noise problem that many of these have. GM has service bulletins about it. If you have the same bearing problem, I could see why it would be different under load. May or may not be it, but another place to look...

[Craig in Denver]

It dosent do it when you just rev it up only when its in gear tho.So, this should eliminate engine problems; hope for u-joints.

[46/71 Hybrid]

It it sounds like it's coming from the bell housing area, it could be a cracked flexplate. They can sound like rod knock and since it's only happening in gear (i.e. under load), I think that could be the culprit.

[46/71 Hybrid]

A couple sites that talk about cracked flexplates and noises similar to yours:

http://www.thedieselstop.com/forums/f31/flex-plate-rattle-198414/
http://www.jalopyjournal.com/forum/archive/index.php/t-189833.html
http://www.dieseltruckresource.com/dev/showthread.php?p=2045325

[deadman1474]

It it sounds like it's coming from the bell housing area, it could be a cracked flexplate. They can sound like rod knock and since it's only happening in gear (i.e. under load), I think that could be the culprit.

We took it to a small shop and the flex plate is cracked and the distrubitor is dieing. I guess when I reved it up I didnt do it hard enough because it starts to do it at 2000 rpm in drive and 4000 in park.

[46/71 Hybrid]

I've worked on more than a few Chevy's, that shouldn't be too hard to fix. If you've never pulled a tranny before, it's not too difficult. If you have all the parts, you and a friend could do it over a weekend, easy.