They do make a bit of an odd noise for the first few seconds after starting when cold. Mine isn't quite as odd/obvious as yours but I can't say I've ever done it with the bonnet up... I suppose they'll all be slightly different anyway. Hopefully you'll set your mind at rest tomorrow with the other one and if anything does develop further then at least you've done the right thing and it's them who haven't at that point. Fingers crossed nothing will get to that stage though.