there is a center in your intake manifold #29221-39600 that has 6 samll flaps and 2 screws per flap they have been known to fall out and boucne around in the cylinder!!
If the noise is coming from the rear then the bushing is broken.
The low pressure port in on the hose coming off of the compressor. It is just under the radiator support on the passenger side.
check heater blower motor blower wheel could be broken off The knocking comes from an Acctuator that has stripped out in the Hvac case. You Have to replace the acctuator to solve the problem.
I get knocking & ping noise coming from the fuel injectors
if it sounds like the knocking is coming from the bottom of the engine i would bet it was the rods or rod bearings may be worn. if the knocking is on the top ,then i would have the lifters and or the rocker arms checked.
low on brake fluid top off
My 2000 Grand Prix would have a low-speed knocking noise coming from the drivers side as the CV axle was failing. The loss of lubrication in the CV Boots allows the joint to make a knocking/tapping sound at a low speed. If the knocking is coming from the engine compartment, then it could be an entirely different problem.
A other small child
Nervous tension.
have you tried higher octane fuel?
Cant you just ask the people at the car place?
If its a legitimate knocking then you would want to go with head gasket if its a tapping you would be looking at a valve