I have a 1995 Centery 18' bowrider with a 4.3 Mercruiser with a Alpha 1 outdrive. This year when we first put it in the water and started out it started making a viberating noise. Almost like something was striking the back of the boat. Once I got to about 5 to 10 miles per hour it stoped until I started to raise the foot to trim the boat out then it started again. It does't do it when boat is trimed out at higher speeds unless I adjust the trim. Also when it is making the noise I can turn the wheel and it will stop. It seems worse at low start off speed or when I try to adjust the foot up to plane out. The boat was only used about 4 times in the last 2 years. It has always run fine with no problems. I have had a hydrofoil stabilizer on it for about 4 years. Any ideas?