If its relief it shold be a whistle sound UNLESS you have the revs down low. If the revs are low the oil will cavetate under load. This will cause damage if you keep doing it. Keep the revs up a bit and the thumping or chattering noise should go away, common for running vane pumps at low speeds, the need the speed to keep the vanes seated correctly.Tha just the sound of the relief valve opening, when the cylinder hit the end the oil from the pump is allowed to bypass the cylinder (that can on longer move, its bottomed out) by going over relief.
Ken