Tranny trouble...

I noticed a strange sound today coming from my tranny i think. The car shifts fine. In second gear, at low RPMS, like 1500 - 2000, when i give it gas and accelerate, i hear a strange vibration/grind/noise. All other gears are fine, and 2nd gear is fine at other RPMS. 2nd gear engages fine as well. any ideas? I checked the tranny mount, and its fine too.

Fluid level is fine, i checked it a week ago or so...although i don't know when it was changed last. would old fluid cause this at all ? When I checked it, it didn't look muddy or anything, although it was pretty brown.