It's possible I guess. However, I've never seen it and I've used the 1N4000 series for over three decades. Your description sure makes it seem like something is having an intermittent arc. I'd venture to say that any 1N4007 that ticks audibly is either being overstressed or defective.
Since you removed the snubbers and replaced the diodes. I'd guess that it's something else and just happens to come from near the diodes. What else is near the diodes?
Does anything you do seem to cause the ticking rate to vary, or is it just getting steadily faster?
Does mechanically damping the diodes by putting the eraser end of a wooden pencil on them damp out the tick?
There is a test which can be very revealing. It requires some setup to do safely. Wait til it's dark outside. This happens once per 24 hours, but if you happen to be having a solar eclipse, that would work, I guess. 8-) Open the amp up and put it on the bench where you can power it up. Carefully note what's nearby you and things you should not touch - like the insides of the amp. Then turn the room lights out. Verify that you hear the tick, and look very carefully for small arcs of light inside the amp. DO NOT ATTEMPT TO MOVE AROUND OR TOUCH THE AMP WITH THE LIGHT OUT for obvious reasons. If it's arcing, you can see the light unless the arc is entirely inside a component, like the power transformer.