Thanks for your comments. Unfortunately the noise I'm getting is purely wind rush noise. There is actually zero perceptible mechanical noise from the fan itself. I might try opening out the holes and/or drilling some new ones to (a) Improve airflow & (b) Quieten it down a bit. I'll also try putting some fine Polyester gauze between the fan & the lid to see if this has any effect. Oh, and then there's the control circuit I want to build so that it only comes on when it's too hot. So many thing to do... so little time...