Police and fire services in the West Midlands were inundated with phone calls mainly from the Coventry area shortly after 6pm.
Social networking sites were busy with users reporting they too heard the noise, some as far away as Swindon and Oxfordshire, with speculation on the cause ranging from a large explosion to a ground tremor.
An MoD spokesman said two Royal Air Force aircraft were authorised to go supersonic after a small civilian helicopter had emitted an emergency signal.
They were already in the air and on their way to the helicopter by the time the pilot realised he was transmitting on the wrong frequency and switched to the correct one. The response was standard procedure after receiving such a signal, the spokesman added.