My woodworking teacher in high school always said you can't fix what you don't see. That Sharpie trick is the perfect example of making a hidden problem plain. It's amazing how many skills come down to finding a way to get clear feedback.

Sometimes a hum is just road noise from new tires or a bad patch of asphalt. I've driven cars for months with a faint hum that never got worse and just went away on its own. Fixing every little sound can lead to wasting money on parts that didn't need replacing.

Learned that lesson the hard way. Ignored a weird hum and ended up needing a whole new wheel bearing.

Remember hearing about using a pencil eraser on sticky relay contacts. Sounded like total nonsense to me until a stubborn safety circuit kept dropping out. Rubbed the contacts with a fresh eraser for a minute, blew out the dust, and the thing held solid. Felt like an idiot for it working.

True, @nina_johnson86. I used to trust public wifi until mine got hacked.