Coming down an NSL dual carriageway last night my 54reg 73000mile WRX developed a knocking sound which got progressively worse. The engine was making normal boost and power. The knocking seemed to happen worse on overrun whilst moving. Looking again this morning, it happens with the clutch in (so it's not gearbox) and at idle. It is related to engine speed and is a high clanking sound not a low banging noise.
The car was serviced 3000 miles ago and there is plenty of oil in it. It's standard, power wise. The car's had pretty heavy use over the last month (3000 miles) and I've covered 1000 miles since Monday driving back to the UK from Norway.
Any ideas what it might be? I'm based near Cambridge so I'd like a garage recommendation other than the local dealer who is extremely expensive.
Cheers!