Knocking in the engine compartment
 

Hello,
I can hear an occasional knocking in the engine compartment, it seems to come like something is hitting the lid just on the back seats.
This is occasional but seems to happen more often on driveway travelling around 100-130K/h gently around 2000-3000rpms. Not depending on road bumps.
I wouldn't probably hear it if not travelling without the acoustic insulation.
I suspect AC pulley, reason, a couple of months ago after mechanic installed the Tarett pulley we could hear some wobbling and noise from the AC pulley, that went away after a couple of minutes running the engine on idle.
I removed the lid and inspect all pulleys manually turning, shaking them, there are no plays.
:confused:

Any advice what could be knocking?