To start I'll give you some history, 1998 4dr 4wd blazer 4.3l, engine replaced last year engine came out of a 2001 1/2 ton 170,000kms on it, new engine had ALL new gaskets installed, new tune up parts all ac delco, new timing chain, new distributor gear, new oil pump, water pump tstat. Engine was extremely clean inside and still is.

To the problem, Recently and first start up after sitting overnight I'm hearing what sounds like a rod knock, last for about 5 seconds then its gone, tried 5w 30, 10w 30 and now it has 15w 40, Noise does not change at all, no engine light, and oil pressure while driving hot is 3/4 gauge, at red light in gear pressure is 1/4 gauge, only thing I can see is the cooling fan clutch has a lot of play in it, noise still sounds quite low, and doesn't last long enough to get a video.

Any ideas? I was thinking maybe balance shaft, or transmission? Didn't really want to spend $100 to find out the fan clutch doesn't fix the noise if some one on here had a better idea

Thanks

 

OK - 15-40 oil not a good idea , meant for diesel engines to contain/combat soot . Your engine bearing surfaces , races will suffer as You are basically starving them . Go back to recommended 5w-20 0r 30 weight ASAP .
Your clutch on the fan is most likely bad . A roaring fan on startup or wobble is not normal. Balance shaft will make noise when engine oil hot and bushings out of spec - it makes a rattley , annoying sound if the engine is revved . I hate that .

 

Cooling fan clutch fixed it, never heard such a strange knock noise from a simple part like that