Jet-engine noise on sleep?

Last 4 or 5 sleeps on my iG5/1.6 have resulted in it powering up the fans 
like a bloody jet engine before it finally sleeps. Last time this happened
(2 
years ago) it was the dreaded exploding capacitors (got the 
midplane/motherboard replaced). Before I power this down for a look-see,
is 
there anything else that might be causing this?

Cheers ... Mark