i decided to drive my 87 t2 yesterday and noticed a crazy vibration rattling noise coming from the in the dash or up in the engine compartment. it only occurs at high rpms and than once the engine speed drops it continues for about 5 seconds afterward. it sounds like sand or little rocks being shakin up. what the hell could this be? thanks for the help

 

Any mouse droppings on the carpet?

Maybe air bubbles in the heater core?

 

could be a leading coil or spark plug wire failing and the trailing firing late (gives a knocking sound and rough upper RPM running).

 

it really could be air bubbles in the heater core cause it sounds like a liquid and it is coming from the dash board area. the car has been in storage so it sits for extended periods of time which might cause those air bubbles u are talkin about. could this really be the problem? and there arent mice in my car, lol.