Help With Engine Noise....

victor231 · December 24, 2007

I am having a problem figuring out a new noise in my 2001 silverado 2500 with the 6.0L 364 with AC. I do have the piston slap, but it's very faint, and like everyone else, it goes away when it's warm. This new noise is very different. It started the other day when I was passing someone. The tranny downshifted out of OD, then all of a sudden this loud sound started, like a whine, almost like the fan was running against the fan housing, or a pulley had just broken a bearing, but there is a definite ticking in the noise that changes with rpm. I backed off the throttle, then started adding throttle again, no noise. It has happened a few times since then, mostly under the same circumstances. The truck is plenty warm, in fact it hasn't done it at all until the truck has been running for atleast 30 minutes. I changed the oil and drained the trans about a week ago, and all fluids check full. I can't see any obvious rubbing marks on the radiator or fan shroud, and the fan and clutch seem to work fine with no binding. The belts seem fine, the upper idler was changed about 3000 miles ago. The truck has AC. I have checked under the hood with the engine running and I can't find any noise at all. It seems to start at about 1800-2100 rpm

It almost sounds like engine pinging, but when back off the throttle and then get back into the throttle, it has gone away. I have even had the noise start when I put my foot in it and then I put my foot in it more, and the noise gets steady and the ticking will increase with RPM, but doesn't really get louder. It is one of those noises that makes it obvious something is going to break, anyone have any ideas? Lifters? AC? knock sensor? I am positive it's not piston slap.

Thanks

ws6_guy · December 24, 2007

hmm, cant be the alternator whining?

victor231 · December 25, 2007

I thought it might have been an alternator, and I have tried to get it to make the noise while I am under the hood, but I can't. It almost sounds like something (like the alternator armature) gets off balance, then stabilizes again when I let off the gas. It's driving me nuts because I can't get it to do it even if I pull over immediately and pop the hood. I'm about to just start pulling all the accessories and replacing them.

Do the lifters in these engines ever go bad that way? Oil pressure is about 40-60 depending on temp and I run 5-30 oil.

victor231 · December 27, 2007

So after driving this thing for a few days now, I have eliminated a few things. I am now thinking that it is valve noise, either detonation or bad ignition. It only happens under load, even if it's just pulling away from a stop sign. Once it happens, I can make it louder or quieter depending on how much gas I give it.

I had a trouble code for the rear knock sensor a few months ago, but the GM mechanic told me to just drive the truck and see if it cleared up, it did so he said don't worry about it. Now my question is that if the knock sensor really is to bad, is it possible that it is bad I am no longer getting a trouble code (the check light does come on normally at startup)? Is there some other module in the ignition that could be failing and not retarding the timing besides the knock sensor?

cbxeric · December 27, 2007

Replace the knock sensors. There are hard codes which will trip the check engine light. There are also soft codes which just register in the computer for reading with a scanner. I am not sure what kind of code a knock sensor will give, it might depend on the type of failure. My rear knock sensor let go a while back, but it was while I had a blown intake gasket, so I got a lot of codes all at once. I found a lot of corrosion on the rear knock sensor (bank 2). Of course it didn't help that the rubber cover wasn't installed properly by the factory. Valve/lifter noise would be constant.

Good luck. Eric.

victor231 · January 1, 2008

I ordered the knock sensors and I will replace them, but a friend of mine and I ended up trouble shooting for about an hour what the problem was. It turned out to be a broken flexplate. The flexplate was replaced about 3400 miles and 4 months ago. We had to get the truck on a rack and move the torque converter with a pry bar to see the crack. One web of the flexplate, and it was flexing enough that the knocking was the end of a bolt on the converter hitting the belhousing, and the grinding was the flex plate on the block.

I talked to my tranny guy who said that he has seen this 4-5 times, but only with the GM mexican made flex plates. He said he is going to check everything on the motor and trans to try and find the cause of the failure, but he said the other 4-5 that he has seen, all going less than 1 year before breaking, got replaced a second time and have been fine.

I tried to find a heavy duty one. I called all the local speed shops (Vic Hubbard, Gotelli's, etc) and even spent time on the phone with TCI, B&M and a few others. No one makes a heavy duty plate for the 6.0 engine with the 4L80E trans. The best I could do was a custom one from a machine shop back east which was about $400-500. The tranny guys opinion was wait and see if he finds anything, if not replace with another GM plate. If that breaks again, he said I should buy the heavy duty one and he will install it no charge.