The most common items that make noise in the rear of PT's are the watt's link in the suspension that Rich mentioned and the rear wheel bearings. Neither of these seem to fit your description of the noise though. The bearing are more likely to make a continous growling/howling noise that is proportionate to raod speed. The watt's link would definately make a knocking noise but would be more sensitive to suspension travel and load than speed. Speed could make it knock more just because of movemnet though. 4,5, & 6 are the watt's link components in the picture. THe bellcrank is the most common part to make a knocking noise in the rear suspension.