Noisy engine

[MORNING LIGHT 1]

Hi , got a Catalina 26 1991 with a Universal M2-12 engine and seems that it began to be noisier recently. I checked the waterlock, seems ok, no leak anywhere, took it for a ride at 2900 rpm and reached 5.9 kn.Everthing
looks fine to me but at iddle, makes a lot of noise.Could the waterloock be at the end of its useful life even though there's no leak . Should I look for something else ? There are 2 air flush valve on the engine, could they be of any utility? Going away soon and don,t want to get stuck somewhere !