Need help finding engine noise.

TennMan · April 25, 2020

I have a 2001 Sierra (4.3 with auto) that has a rattle that appears to be coming from under the throttle body. It sounds like a muffler clamp that is sightly loose and vibrating on the exhaust pipe. The noise only occurs when the transmission is in gear and the RPM is between 1200 and 2000 under light to moderate load, It's annoying because it occurs when accelerating normally from a stop when it hits that RPM range. It's not too loud but it's running me crazy.

It doesn't sound like a spark ping or a rod knock or a tick like a lifter. It sounds like a rattle that changes in intensity randomly as you accelerate. It won't do it every time and it will go away if you back off the throttle and get back in it... sometimes but not always. I can put the transmission in Park and engine sounds normal and quiet as a mouse while holding it at any RPM.

I have only owned the truck about 2 months and the whole time I thought it was a catalytic converter that was rattling because the insides had came loose. I've had that happen on other vehicles and that is exactly what it sounds like. I took it to a muffler shop to get the converter replaced but the mechanic, who has changed hundreds of converters, put it up on the rack and he said the rattle wasn't coming from the exhaust. He said it was coming from somewhere in the engine toward the top. I have to agree with him. I can make it rattle if sitting still with my foot on the brake and load the engine by bringing up the RPM to about 1500. I had my wife hold the brake and bring the RPM up to load the engine while I listened under the hood. The rattle sounds like it is coming from under the throttle body.

Any help would be greatly appreciated!!!

rjgti · April 27, 2020

egr

rjgti · April 27, 2020

timing chain

TennMan · April 27, 2020

Thanks for the reply. Can I tell if it's the EGR by unplugging it? How check the timing chain without replacing it?

rjgti · April 27, 2020

no, unplugging will not work it must be removed and inspected. a timing light to check for timing chain

https://itstillruns.com/check-timing-chain-7521157.html

TennMan · April 27, 2020

I'm going to take the EGR off today. Can you tell me what to look for when I get it off?

TennMan · April 27, 2020

I swapped the EGR from my 4.3 S10 Blazer that is running great with the one on my truck. It made no difference in the sound. I put a stethoscope on the timing chain cover and the rattle is no louder there than it is when putting the stethoscope on the intake manifold.

I took it to a mechanic who has been in business for years that has an engine rebuild shop and had him listen to it. The good news is he said the noise doesn't sound mechanical to him. He said since it only does it under load he suspects it is a spark knock or something loose on the engine rattling.. He told me to run a tank full of the highest octane gas I could find through it and see if the noise gets better. If it does he said to change the knock sensor. I would go ahead and change the knock sensor to rule it out but it is hard to get to on my truck. It's in the rear and really close to the firewall making it hard to get your hand on it. I'll try the high octane gas first as he suggested.

rjgti · April 28, 2020

are your ks not under your intake.

TennMan · April 28, 2020

12 hours ago, rjgti said:

are your ks not under your intake.

I don't think so. Yesterday the mechanic showed me a 4.3 block he had sitting in the machine shop. Iit had a threaded hole on the block just behind the left cylinder head where the knock sensor bolts on with one bolt. On my truck the firewall is just a couple of inches (or less) away from the block in that area. I can't get my hand on it from the top. My hand would have to go down behind the valve cover and the head to reach the sensor and my hand just won't fit. I'm going to jack up the truck when I get time and see if I can reach it from the bottom. Thanks for your suggestions. I appreciate it.