I started to get that slight howl on my Exedy Twin HD after about 1000 miles on the clutch. It would happen only in 1st gear when I disengaged the clutch from a standstill. I adjusted the clutch pedal a little and it went away. It eventually came back after another couple of thousand miles. I didn't want to adjust the clutch any more after that. I just ended up living with that noise. Now I'm getting my clutch rebuilt after 2 1/2 years of use and am curious if that howl will return.

 

Bump for an update to see if the OP ever found out his problem, I fear I have a similar problem.

 

its normal. mine does the same thing.