Had my first glitch on Friday when I heard what sounded like a twittering noise coming from the offside rear wheel when driving through the Heathrow tunnel. It started again after about 50 miles on the return journey and passing through Pulborough the XF was turning heads for all the wrong reasons! On Monday the noise started after 25 miles, indicating the problem was getting worse and heat related. When I took it to the dealer on Tuesday morning the rear offside was twittering like a skylark after 18 miles so I had something for them to go on. Unfortunately the car sat for 30 minuteds before they looked at the brakes and could find nothing wrong. After a quick drive round the block they failed to induce the noise so they asked me to take the car out with the mechanic in the back seat so he could hear the noise if I could get it to start. After two trips round the block it started again and I was able to show him how light braking silenced it. He removed the rear offside caliper, cleaned and lubricated the pistons, which were not returning home fully when hot, forcing the pads to stay in light contact with high spots on the disc. Touch wood everything is now back to normal.