If it does it all the time get it into a shop and have them look at it. The dealership I took it to put some washers inbetween the timing belt cover and the engine. Cheap way out, but got the job done.
'95 Contour GL, zetec mtx, drop in K&N, exhaust res removed, 4 10" Kenwoods powered by 1000W Visonik amp, 4 5"x7"/6"x8" Visoniks