Sorry for the delay , it was in fact a piece of sponge as said earlier, that had been used to stop debris and dust getting into the stationary engine.
looking at the front of the engines I was to the extreme right using a small hand held Vid camera, as I looked up to see with naked eyes I though it was a hat. but no damage done, thankfully.
But that vibration in the air cannot be explained it is totally different to the air vibe given out by the Merlin or the Griffon, I took my old dad down to the Fighter collection about five years before he died, we were giving them some Starters and Mags for Griffon engines, and arranging for my driver to bring down this very R/H Hercules so they could use it to accurately pickup the engine mounting points from the Herc so they could build the engine mounts into the Beaufighter wings, they had already built the cockpit section of the A/c and had quite a few instruments in their and a control col, whilst I was busing myself peering into exotics like the Mustang and I think a P 47, what I only just noticed was Dad ashen faced and with one hand on the cockpit, with eyes so full he could not see, I took hold of him and walked him towards to door this was also seen by the young man showing us around, he quickly suggested we walk to the tie down area out side to watch the first full power run up of a rebuilt Spitfire, that calmed things down a little and I sensed it was time to go, during the next few years Dad unloaded stories to me that he had kept bottled up since those dark times, I feel that four years allowed my beloved father to have the ability of a peaceful and unburdened end to his proud and busy life, I do hope you understand what I have written here
Peter R-B