I have a 2001 w/ 5 speed manual, only aftermarket thing is full exhaust and I've developed an odd vibration. Not sure when it started but whenever I hit 4k rmps, whether its in gear or not i have a vibration that feels and sounds like something is being rotated through gritty oil. It gets worse the faster Im going (5th gear is worse than 4th and so on) but its only at 4k rmps. Nothing over or under. any ideas on where to start looking?