Rev counter

Hi there when i rev my car the rev counter increases like normal but when i let go of the accelerator the rev counter drops to about 2000 revs then slows right down and takes ages to get back to about 1000 revs, also there is a noise coming from something i dont know anything about when the revs drop, as if there is something stopping it dropping back to 1000 revs normally, there is a picture attached, if anyone could help i would be very grateful, many thanks, burt