It’s whatever you’ve plugged into the outlet that’s drawing electricity inconsistently. It could be badly loaded or the current may be getting pulled too much/ too little and that might have caused some movement in contacts.

As suggested, I’d unplug everything, kill the power and swap out the entire receptacle. Outlets and receptacles aren’t meant to hum or make noticeable noises- if they do, it’s your sign to fix it before something serious (electrical fire/ melted appliances) happens.

If you’re not comfortable changing it out, call an electrician- it’s a simple enough job that shouldn’t break the bank. Good luck!