As the topic says, my brother has a 00 Si. Whenever he reaches 4k in any gear, u hear a grinding noise. What could it be?

ps. its only at 4k

 

does he drive the car hard? if he does the gears might b worn out and he prolly needs to replace or rebuild the tranny. Also whens the last time he changed the tranny oil? If the tranny oil is black and has lots of metal sediment then the gears are getting worn out.

 

We'll check that tomorrow. Oh BTW, his car used to be boosted. But its wierd, only at 4K it makes the grinding noise. If it goes past 4K it stops.