Gerry, you can check some measurements if you want:
Action in place, measure distance between hammer at rest and string. On your Bösendorfer it should be 46mm. If it's much greater than that, say 50-55mm you could have a problem of:
Distance between hammer shank and hammer rest.
Action out, you'll either see:
*a white cushion of felt under the hammer shank, just before the hammer head
or
*a long strip of green felt under the hammer shank, just before hammer head
With hammer at rest, there should be a distance of 3mm between the hammer shank and the 'hammer rest' on Bösendorfer. If the shank is touching the felt, it's wrongly adjusted. If the distance is too small, the noise you hear could be the shank bouncing on the felt. It could make some sense, since you have new, heavier, hammers installed, and also explain why the problem wasn't there before.
The measurements are from a reference book I have. They are of course a little bit of give and take, as the technician must make all things cooperate.
If this is the problem, the technician can solve it in a few different ways. Increase the distance by shaving off felt, or raise the hammer. Or, the repetition lever spring needs tightening in order to better resist the load of the falling hammer. However, the spring needs to be really well regulated for another purpose. But if it wasn't checked and adjusted, it could actually be a bit to loose, since you probably have heavier hammers now.
Well, just a few thoughts. I'd be really interested in what you see when you pull the action out!