What happens when you rev it? Are you definitely sure it is top end?
I was convinced mine sounded like top end but was really hard to locate. Mine was like this:http://www.youtube.com/watch?v=QmD-Z0y7VIM
and turned out to be a big end bearing on number 1! Came on very suddenly and stopped using the car. Only started it up a few times to try to diagnose it but when I took it apart it was like this!!:http://www.youtube.com/watch?v=bQ1Um9iJGnc
Hope for your sake it isnt but speaking to a local mororsport company who use the 3S in race MR2's and in monocoque cars, apparently they can suffer with big end issues like this and they have seen quite a few the same!