My shifter seems to be the source of a very annoying rattling noise.  The sound starts when the motor hits around 2200 rpm and perssits through the gears until 5th.  Even then at around 3000 rpm and 65-70 mph the rattling continues.  The sound is like if one put a few dimes in a glass and set it on top or somethjing vibrating at a high frequency. (if that makes any sense).

Any ideas on what is causing it and how to go about fixing it?

Imdy

Perhaps it would help if I told you Make/Model...
2006 Suzuki Forenza 5speed Manual. 

Imdy

Usually, that indicates to me, that your transmision is about to DIE.   I'd drain the tranny oil and see whats in it.  drain it into a clean pan, and see what you find.   any sort of metallic material= gearbox REBUILD or at minimum a substantial internal problem, like a bearing or gear +/- 3 is in need of a medic.

A rattling shifter on a FWD car is more likely to be a loose shifter linkage than a transmission problem - so time to inspect the linkage and tighten up loose bolts & replace any worn parts.

By the way - this forum seems to have more of a 4WD focus than FWD - you might get a better response over at http://www.suzuki-forums.com/suzuki-forenza-reno-forum/, it's not that you're not welcome here, just that there's more "car related" activity across there.