It's about two years that I can hear a clack-clack sound coming from the engine bay: this appears only in a certain rpm window (2000 - 2500 rpm).

This happens only whith a low load: it disappears in decel and acceleration. It's there only when I keep the speed constant, I have to "aim" at it to make it noticeable.

It's much more noticeable when the engine is cold, once warmed up it's much more difficult to identify it between the other "normal" noises.

Without a certain load it's impossible to hear that specific noise, so I have to drive to hear it. At idle I can't spot it, not even using a sthetoscope.

It's not an accessory (I removed the belt).

I disconnected the injectors one by one hoping to identify something, but the noise was always there, same rpm window, same intensity. This is a procedure described in the service manual. They just say to disconnect the coils. For me it was easier to disconnect the injectors.

The engine a 98 internally stock LS1 (85'000 miles), the only mechanical thing I changed is a ATI harmonic balancer (it drives the supercharger).
I use a synthetic 0W40 oil.
No misfire, no codes, no KR, the fuel trims around 0%.
Right now the supercharger is disconnected and the program is 99% stock.

What else can I check before I start taking the engine apart?
What could it be?
Rollers? Pushrods? Lifters? Timing chain?

If it was one of those parts I think I would hear the noise at idle... or not necessary?
Any idea?


Thanks

Stefano