Stopped inflating, and the noise level dropped a few dbs. :)
Took 4 screws out and opened it up to discover that the tiny compressor con-rod had somehow slipped off the spigot on the cog which drives it.
It was easy enough to squeeze it back on using mole grips, would have been quicker if I hadn't tried with pliers and G-clamp first.
So if yours stops inflating, don't throw it in the bin.