Noise on startup

[kton2002]

Had an oil change done and thermostat looked at, 2.0 petrol all done by a bongo specialist. On collecting the bongo , started it up and as I accelerated I heard a bit of what I can only describe as exhaust blowing noise, but maybe it was a belt sort of chattering. It happened 3 or 4 times, each time was as I started to push accelerator in drive. It didn’t do it at idle at all. Been fine this morning and a few cold starts since. Maybe was a coincidence , can’t see how an oil change or thermostat inspection could have caused that. Only other thought is the under tray or splash guard, seemed a little loose a few of the bolts and washers not all the way in, maybe those vibrating. Or a drop of oil spilt on belt ?