There is a low level noise when braking (about 330 Hz)

Brake pads are a good suggestion and clearly something to check out. However, if the brake pad wear indicators are the source of the sound would you not expect to hear the pitch (frequency) of the sound change with vehicle speed. In other words, I would expect the pitch (frequency) to change from high to low as the vehicle slowed to a stop. For my vehicle, that is not the case. The pitch stays quite constant at about 330 Hz and persists for a second or two even after all pressure is off the brake and the vehicle is barely moving at a crawl. I was wondering if the problem might possibly be related to an issue with the anti-lock braking system.

Brake pad wear indicators typically sound like 'fingernails on a chalkboard' , metallic scraping, NOT so much a 'howling' as you say. What other conditions can you describe, that either aggravate the symptom or change it in any way?

Was this one of them? https://youtu.be/Z3N9SxqKubQ This is just a flat line tone .. Sounds much better, or more convincing, from a guitar! .. Oh crap, I forgot - I don't 'really' know how to play the guitar! Kinda hard to beat the well trained ear of an 'experienced' mechanic .. Suggest have such mechanic test drive since it only does this while vehicle is in motion .. Hope everyone had a nice Thanksgiving, enjoy the rest of the week-end!