Disturbing Manual Transmission Symptom

[Jehu]

I've noticed a few times very recently when downshifting from Fifth to Fourth that when I resume acceleration I feel a "slam" after a second or so.... I remember this feeling from an Auto Tranny in a Van I owned during acceleration i began feeling this horrible slamming into gear. Its pronounced but not horribly shocking as it got in that Van. Any ideas what may be happening? Doesn't seem to happen at any other time and it doesn't happen every single time I downshift into fourth from fifth especially if I do not accelerate immediately. If I let the speed remain even for a few seconds then accelerate this "slam" was not occurring. All upward shifting is smooth and precise.

[Blitzkrieg Bob]

Guibo?

[Jehu]

I'll have a look. Thought occurred to me as well as Tranny and motor mounts or u joint.. like to brace for the worst then if I'm wrong am relieved ,if right then prepared.