Surely a household vacuum is as noisy as an angry EMB145 and quite powerful. At LHR we still do a number of airstarts as a/c are permitted to carry over u/s apu faults. Recently started a 757 with 20psi as the airstart was not functioning correctly and we had already dispatched a u/s one. To minimise noise only one engine started by the airstart the other cross feed on the taxiway and there is a lot more smoke which I assume as the punters can hear the airstart the pilot probably tells them what is going on.