Boat ran fine last year and didnt make any noise.
This year i replaced all the plugs and a couple had a lil rust on them and changed the oil.
Started boat and could hear a ticking sound from the port side of the 502. I went to back of boat to see what the exhaust looked like at idle and the port side pipe had less exhaust coming out of it. The motor seemed like its firing on all cylinders, i plan on pulling valve covers off tomorrow, just was looking for some ideas of what to look for. I bought this engine from a guy and it had been sitting for 2 years. I put it in and it ran fine with the oil and plugs in he had. Plugs looked like hell when i pulled them out and oil looked pretty bad. I changed them to ac1 and royal purple that was suggested on this site. The engine is a 2000 502 and its in a velocity. Once i installed engine last year i might have ran it in driveway hooked to water for 20 mins maybe. Also this winter it might have gotten to 30 degrees where i am at twice but i drained water from plugs and out drive, and i had a heater running inside engine compartment on any night it got to 35 or so degrees

Get Rid of the Royal Purple. Its Junk

sometimes the ticking noise can be a leaky exhaust gasket.
now if the rust was really bad,it could be on the valve stems and its binding the valves up a touch.

I felt around port manifold and didnt feel any exhaust leaking. Im going to try again here in a bit

Hydraulic lifter not pumped up yet? When I replace the 502 in my old boat last year, I swear it took 45 min to get fully pumped up and quiet down on the new to me used engine.

After doing lots of research on this site, royal purple was with in the top 2 pics everytime, what do you run and why do you prefer it over royal purple.. ( just want to educate myself)

I lost big end of rods and mains on cranks. Poor upper end lube. It thins out way to much with heat. And it shot in less than 10 hours. I run Rotella T 15-40 no problems

Make sure you didn't have a spark plug work its way out a bit. Sounds like a to good to be true deal however it does happen and will make a similar sound. Just thought I'd mention it. otherwise the lifter???

Most my experience is when I start to hear a noise it doesn't end up well. Got lucky a few times with broken rocker arm studs though.

maybe that dirty oil had an additive in it to keep it quiet, hope not good luck