I recently purchased a Key West 27' CC with twin Yamaha 150 four strokes. I found out after the purchase that Yamaha has had problems with a harmonic balancer in these motors 2006 - 2009 I believe. The symptoms are a whining sound in the front of the motor that is very evident with the calin off. My port side motor is making this noise. I spoke with my local boat dealer that deals with yamahas and he told me that Yamaha has supplied the parts in the past and the customer just has to cover the labor. Has anyone dealt with this?
This^^^^^^^^^^^^^ Let them run the serial number and see if yours have been replaced. There can be other whines that are not the balancer. It is not a big $ item if you have to do them. Just find an honest Yamaha mechanic.