ive got a 94 wrx been driving with a dodgy maf sensor for a wile now the engine makes a grinding noise when you turn on and of only lasts about a second no noise when you rev and no loss of power but sounds like a bearing is breaking down when you break does this sound like third piston melt

 

if you know the maf is dodgy why are you driving it at all?

 

my car sounds like a bag of spanners when its cold. I didnt drive it for days thinking it was knackered. When its warm its fine.
Doesnt bother me anymore. If the noise clears when warm then i wouldnt have thought it was serious.
Get the maf sorted ASAP though. You will know when its dead as it will knock hot and cold and there will be loss of power (knock correction on over-time)

 

sounds like you've killed it driving it