I drive a 1988 V6 Formula Fiero with 175 000 Km on it.

I have done an engine flush before the last oil change, and add a Bardhal 2
after it.
Since then a bizarre noise come out from the top end of the engine under the
plenum. Sound like some broken ball bearings.

I can clearly hear it putting my hear on the plenum.

I think there is no moving or rolling parts under the plenum.

What could it be ?