Need help with new engine

[NathanY]

I installed a new 5.7 in my c2500. It seems to run alright, but its making a funny noise on the top front of the motor. It sounds like thunk thunk thunk, like an air leak or something, not like metal noise. I am at a loss. My mechanics at my shop are at a loss. We are diesel mechanics, not car mechanics, so we rarely work with gas engines. It almost sounds like an exhaust leak but its at the front of the motor in the thermostat area. I sprayed some carb cleaner around the intake to check for vacuum leaks, and nothing. I am at a loss on this.

 

[NathanY]

Oh yeah, it seems to get quieter as the engine warms, but it does not go completely away.

 

[badazzbulldog]

vortec or tbi motor what year is the truck?

 

[NathanY]

Its a 1999 C2500 5.7 Vortec. Just for the hell of it, I took the serpentine belt off and started it. There was no noise at all, just a smooth running engine. I put the belt back on and the noise came back. I dont know what it could be. New water pump, new alternator, new fan and clutch, but something is making a thunk noise.

 

[glfirefighter144]

Take the belt off and spin the pulleys by hand, other than the crank they should spin, some easier than others, but you may feel or hear which pulley is bad that way

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[badazzbulldog]

check the tensioner pulley also does it have a/c? i've heard the compressors make some funky noises