Groaning clutch.

Hi Folks,
What typically causes a clutch to groan as it's engaging? It seems to work fine but when I accelerate hard from a stop something groans until I'm completely off the lever.
Thanks in advance.
Bill

Mine's been doing that for years. I think it started when I overheated the clutch then had a sticky cable. Replaced the clutch and cable but it still does it sometimes. When I did the tranny I looked closely at everything finding nothing. It would slip in 2nd on hard takeoff after my port job. Wound up replacing plates, cable, got a roller pusher and added an extra plate and a late style actuator. It only does it once in a while now and the only thing I haven't replaced is the needle bearing on the clutch hub. That could be the culprit. 
Steve