I had the same noise a few years back on my old bugeye.
- was my clutch on its last legs - few days before it died.

If your clutch is ok I'd be checking all bolts and sheilds (put your head under the bonnet and get a friend to rev it/start it do what ever)

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1989 Legacy GT --> 2001 GD WRX --> 1999 GF8 WRX --> 2001 GD WRX