Knocking sound in gear

Every time I start the car, put it in gear and move, I hear a dull knocking sound around where the clutch is. It stops in about 20 seconds. This happens mostly in reverse, also in first gear sometimes and especially after the car hasn’t been used for more than 6-7 hours. Any idea what this could be? Thanks in advance.

Champak
(from New Delhi, India, the city of potholes, dust and heat)