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Noise-tapping when accelerating

Truck: 1993 Chevy C-1500 Half Ton, 4.3 V6, Automatic transmission

Problem: When accelerating there is a loud tapping sound from the engine area. It sounds like a metal part. I can drive about 1/4 of a mile before it starts. I recently replaced both motor mounts and, later, the transmission mount. I thought the oil pan may have been bent inwards when doing that job, but the noise did not start until a week or two later. The transmission mount was replaced after the noise started. The noise sounds like something hitting the bottom of one of the pans and does seem to come from the lower part of the engine. I cannot get the sound by revving up the engine while in Park. It only does it when it is actually moving. Because of that I ruled out, possibly incorrectly, an exhaust leak. I have also ruled out the fan hitting the shroud, because I removed the entire shroud, drove it a short distance, and heard the noise. The noise seems to follow the movement of the truck (e.g. when I'm accelerating there are dips in the road, the up-and-down movement causes the sound to get louder and quieter in line with the truck itself). I have not noticed any problems with oil pressure or temperature.

I know this is a long winded explanation, but I really hate taking my vehicles to a shop. Any ideas would be appreciated.

ATL

Reply to
ATL
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How many miles on the motor? If you start it up (engine dead cold, like first thing in the morning) and drive off right away, does it start making the noise right away? If not, how long does it take (minutes, not miles) before it starts tapping? Drain the oil with a magnet sticking into the stream of draining oil. Any chunks? Also change the oil filter, cut open the old one and look for the same.

It could be as simple as spark-knock, or it could be a spun bearing.

Doc

Reply to
"Doc"

Does it do real bad when accelerating and turning left? I see you've already replaced the motor mounts and ruled out an exhaust leak but I had a

92 that did about the same thing you described and it took me over a month to figure out I had an exhaust leak where the manifold connects to the pipe on the driver side. I had bad motor mounts that caused the engine to rotate and pull the manifold apart from the pipe. Check to see if you have any soot around that area and check to make sure the bolts are tight and the springs are good.
Reply to
Goff

Doc, Thanks for the suggestions. I will try them over the weekend.

Truck has over 115,000 miles. The noise is not heard when I first start driving. I can drive about one or two minutes before I hear it. It seems to possibly be related to the temperature. Once it warms up, depending on the outside temperature, it begins the tapping.

Thanks again, ATL

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Goff, I don't think turning makes a difference, but I will start paying attention. The very idea you suggested had crossed my mind. I might replace the gasket at the joint you mentioned, just to eliminate that possibility.

Thank you, ATL

Reply to
ATL

Sounds like it's leaning out when it switches to closed loop.

Doc

Reply to
"Doc"

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