Have you check the Prop? Look into the pump area and also lay on your back up under the ski and look into the grate making sure their isn't anything lodged up their, had a customer with a stick stuck up in his. Kept telling me about the same thing, then I layed up under the ski and pulled the stick out that was lodged in the pump, as he gave it more power it bowed or something and made a terrible growling noise. Make sure you grease the drive line also at the grease zert.