You have answered your own question, take to be serviced/repaired, assuming they are reputable.
I don't really understand what the problem is as you have not described the problem very well and haven't even described the kind of bike.
Beware the 'avid biker' if he was really knowledgeable/ helpful he would have offered more than "might need a good lube".
Assuming its an ATB, maybe the ringing is coming from the rotor, if you had a loose spoke you would feel it when you pluck them, and what actually rattles?
How do you know the bike was "tip top" and what was serviced for only £20?
But its fair to say,reading between the lines you are not going to solve the issue, so taking it somewhere you are confident the job will be done properly is your best option.
I am assuming again here but, You have a Carrera ATB, have given it some use, took it in to
Halfords? didn't really know what to ask for, they gave it the once over, tightening the bolts and pumping the tyres up etc and charged a modest £20. What they didn't do was attend to the issues due to wear and tear because they are not up to it?
If I got that all wrong, my apologies, just seen it all too often before, but get it sorted and ask the shop questions as to what they have done to put it right.