2.3L quad OHC double-knock at startup

94 grand am 2.3L quad OHC with 90k. When i startup- hot or cold -there is a distinctive double knock from the engine compartment. Sounds like its coming from under the quad cover. No noise though once the engine is running. Any clues as to what's the problem? thanks in advance.

history
bought in 96 with 11k
oil changes every 3 k, trans every 30k
coolant changes every 2yrs
tyres and brakes changed twice
new front and rear suspension at 80k
runs and looks great, so far..