Strange Sound on startup.

kobicalvert · July 10, 2014

I need some help! I have a 2007 Chevy Tahoe 5.3L with just under 190K miles. Lately (about once a week now for the last 3 weeks) when I start the truck I hear a rapid tapping sound similar to what you'd hear if you tried to place a ruler or stick in the radiator fan. The sound only lasts for about 5 seconds. I've tried to reproduce the sound for diagnosis without any luck, it only happens on occasion as stated above. Each time the noise has manifested, the Air Conditioning was on. I've heard of a phenomenon called "Slugging" which is caused by a foreign fluid in the AC compressor, that could be the cause, but I've yet to be able to confirm this. I looked in and around the radiator fans, and there is no play, nor is there anything the fan blades could be hitting. I've also heard this type of sound could be a water pump getting ready to go bad, but I've read that is more of a grinding sound. Anyone else ever have this problem or have heard of anything even remotely close? I do not believe it is valve tapping, or any type of knocking/preignition. The oil and coolant levels are full. Any and all advice is greatly appreciated. Thanks!!!

efritz · July 10, 2014

Some folks have had issues with the AFM lifters knocking on startup. It can sometimes be traced back to an oil filter with a bad anti drainback valve. Change oil and filter with a quality filter and see if it helps, just a cheap thing to try.

kobicalvert · July 10, 2014

Thanks for the reply, I did try that and it hasn't seemed to help. ;/

gmtech4 · July 10, 2014

foreign fliud ???? what are you talikng about ???

water pumps dont do that ,,,,

youve been given very bad ideas sir ,,,,

get an audio clip and post that ,,,,,anything short of that is BS guessing (happy face )

kobicalvert · July 10, 2014

Hi Senior Enthusiast, go to youtube, and type in "signs of a water pump failure" and you'll be able to hear the pump making a grinding sound due to bearings going bad. As for the foreign fluid, this is not my specialty, but is it possible that water could get into the compressor or more specifically, moisture collecting internally, thus; a foreign fluid. If you read my post carefully, you'll see I've been trying to reproduce the sound so I can get your clip. And lastly, it isn't all BS guessing, especially if someone has witnessed the same problem, that is why I'm here, looking for help.