"ba-hum-bam" or letterboxes to you and me.
Causing a bit of a problem though as he's obsessing over it so we keep being dragged off to the front door so he can look through the letterbox (more a compulsion maybe than an obsession). And of course screamed at if we don't do it.
Still I guess its less destructive than the toilet flushing one (see small bore mashy toilets in other subjects for the problems with that).