Whats that noise???

[CaptainRandall]

I'm taking a stab in the dark here, hoping that someone can help with this. I have a 'noise' happening within my dashboard. It sounds like a low-pitched alarm. It's more like a rattle or buzzing sound. I thought maybe it's tied to my bilge, but it has happened when the bilge was dry. It's not the sniffer as that is a high-pitched 'tweet' and does not stop when I push the mute button. It has happened while underway, while connected to shore power, while no shore power in the middle of the night etc. There is no rhyme nor reason as to why this is happening. I had my head up the bottom side of my dash for half a day looking for anything out of the ordinary but can't find a thing. Help!
Randall