This sounds like engine knocking to me. You notice this sound at low
rpms and during acceleration where engine load is high. If your engine
is fully computer controlled you should review your knock sensor. It
may be not working as it should so the timing cannot be retarded. Check
it again by fulling your tank with a high octane fuel.

If during acceleration it is likely detonation, otherwise known as ping,
spark-knock or east of the atlantic, pinking.  If the car has detonation
detection, the detection system may not be working properly.  Or, if it
does NOT have detection, it may be a fault in the spark timing.  To see
if it is spark knock, try a tankful of the highest octane gas you can
find in your area, and see if it eliminates or at least reduces the knock.