Pulsing cat

This is an odd one: My cat is 7, and recently I noticed the following: to the left of his tailbone (or end of his spine), about 2 inches in from the base of his tail on the left side of his hind quarters, there is an irregular "throbbing" or "pulsing." I notice it when I hold him, and when he sits with me. Sometimes, it pulses 1 time, sometimes 3 or 4. It's pretty constant, occurring every 10 to 30 seconds. There is no pain or swelling, and he does not even notice that it is happening. Have you EVER hear of this???