Loud metallic rattling just before air pressure release?

[JamesH]

So I noticed that the engine (8.3 Cummins) or air pump, not sure which, gets really loud like out of adjustment lifters as the air pump reaches max pressure and the relief valve opens. Gets nice and quiet for a few minutes then starts to rattle again, maybe a minute before the pressure relief valve dumps air. Is this normal?

 

[mcmullag]

Mine does that, I think it is normal. I think my previous one did it too.

 

[Ford Mechanic]

Mine gets louder also above 100 psi. Almost sounds like the compressor has a rod knock. I happen to have other trucks (not MV's) around that have the 8.3L and they all have similar, increasing noise levels over 100 psi from the compressor. The odometer range from 140k trash trucks that run 6hrs X 5 days a week, to bucket trucks with less that 20k (lots of stationary run time). I belive it to be normal. I've had mine for 2 yrs, it has 18k on it and I've put about 2k on it. I noticed it the day I bought it and actually used it against the truck during negotiations. It hasn't changed a bit.

 

[dawico]

I can always tell when my compressor is pumping because of the slight knock it makes. Mine goes on and off frequently because my compressor is leaking (for now).