- gordonbennet
Braking Bad.

Unless you are travelling on unusually gritty surfaces, the chances are its high frequency vibration of the pads causing the squealing, it's not usually the pad vibrating against the disc, but the pad vibrating against the piston or sometimes the edges of the pad backing plates rubbing on the edges of the caliper.

The pads need to come out (highly likely the dealer only squirted brake cleaner at them) and be inspected, upon which there will probably be a shiny section or two to show where this high frequency rubbing is taking place, the answer is a proper clean up followed by the application of the correct brake grease on those parts.

Many mechanics give a wipe of coppaslip on the back of pads when they are fitted, this helps but coppaslip isn't good for rubber dust boots if it comes into contact over a long period, hence correct brake grease is better which doesn't affect rubber.

Whilst the pads are out its a good opportunity to generally lube where necessary all pads, sliders, pistons etc, as well as wipe of normal grease on exposed corrosion prone brake pipes, something that should be done at least every other year, a service most maker's dealers don't perform (unless you are a home mechanic i'd always suggest a trusted indy for such tasks), done properly will see calipers last the life of the vehicle as well as keep the braking system efficient.
Honest John's Motoring Agony Column 16-11-2019 Part 2 - Brit_in_Germany

The thought of putting "lube" on the pads makes me shudder. Also, putting grease on the sliders will only allow muck to accumulate there and is not a particularly good idea.

Honest John's Motoring Agony Column 16-11-2019 Part 2 - MoDo613

I don't think he means putting lube on the friction surfaces! Grease specifically for brake pads, or copper grease is regularly used on certain parts of the caliper/pad assembly - obviously not the friction area.

Honest John's Motoring Agony Column 16-11-2019 Part 2 - Brit_in_Germany

The previous paragraph talked about putting brake grease on the back of the pads though.

- Eudo H.
Braking Bad

At a toll in France I did five lengths next to a Q7 whose brakes squealed aggravatingly loud. Like you, I had to wind up my window. Small comfort
I know, but you are not the only one.