H.O. Alternator

[tsmf509]

Scratching my head on this one...I just installed my 370 Amp Mechman Alternator. Along with that I changed the belt tensioner and went down .5" in effective belt length. The problem I'm having is while the car is placed into gear (under load) after start up and at idle rpm. The belt vibrates/ flaps. However, it doesn't do it upon start up, with HVAC on or when you rev it. Only when placed into gear. I spoke with the tech and he says that if my battery voltage is below 12.6volts then that's the cause. Which still doesn't seem right. So I've since played with different belt sizes to see if there is a change. The vibration/flapping stops after the battery is at the proper voltage. I still suspect a bad belt tensioner though. Any input is greatly appreciated. Just FYI....This is the third vehicle that I have that I've i stalled a 370amp Alternator on and the first one that has given me an issue. I've also got a XS 3400R enroute.