My 98 PA did the same thing until I figured out what it was. The front coil
springs sit in a rubber bushing that can get hard and dirt can accumulate on
the top of the rubber. I jacked the car up in order to raise the last coil
of the springs off the rubber as far as possible and then sprayed the area
with silicone real good. You can do this with out removing the wheels. With
the car jacked up you can reach in just above the tire and spray the entire
area. I do that a couple of times a year and no more knocking when going
over bumps. Mine did it mostly in the winter but doesn't do it at all any
more. Hope this helps.
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