My JVC RV-NB20B makes a loud humming noise sometimes when it's first switched on or if accidentally knocked. I'm guessing something is loose inside as it is an intemittent problem, but what..?

SOURCE: Enegine has loud knocking noise in 2000 Honda Civic EX

maybe its low on oil

SOURCE: 2005 jeep 5.7l Hemi

It could possibly be "detonation" knock .. try using some premium (high) octane fuel and possibly an octane additive for a tank or two ..

SOURCE: Knocking noise from front when pressing the gas

This is a rod going out.

SOURCE: knocking sound from wheel well

it sounds like you have a bad cv axle on the drivers side.

SOURCE: 2002 trailblazer making loud humming noise when driving

When you turn right - you're taking weight off the right wheel.
If the noise dissipates - the passenger side wheel bearing is most likely to blame.

Thanks

Testimonial: "thanks, wish we would have known that before my husband spent 3 hours on it yesterday..it does make sense tho..thank you VERY much for your help"