Diesel heater has started to make a wierd noise.

My heater a Vevor 5kw that is located in my engine room on my boat, has been running flawlessly for 6 months now. But suddenly i hear a strang noise from the engine room. It sounds like a vibrating/humming sound every 10 second or so. It’s not a high pitched sound. My guess is the fan but i could be wrong.

Any ideas ? Should i be worried ?