I call them thumb farts.
My Voyage is 14 months old and the feel seems to change sometimes. Sometimes its more of a thunk thunk, sometimes a bit of a vibrate, although more of a vibrate of the case. There are times when I can barely notice and just get a very faint ump ump. And I always had the setting on low. It just seem to vary a bit. Didn't bother me in any case, although I did have it off now for a month after it went to the faint ump ump again. I couldn't feel it, even using the high.
Now I turned it on to low again and its just fine, very noticable and doing its thing. I think mine is a bit moody.