I have a 1998 Camry V6 with 165000 miles. I had brake pads and rotors replaced on all four tires. Two days later, I noticed a flapping/knocking noise apparently coming from the rear of the car. It only happens after the car as been driving for 5-10 minutes. The noise increases in frequency as the speed of the vehicle increases and is unaffected by braking or bumps and gets louder the longer the car is driven. I took the car back and they re-checked all the brake installations and said everything was fine and the noise was nothing to worry about (of course I had to convince them that there really was a noise). They also said they inspected the tires and didn’t find anything wrong. I have jacked the car up and looked under all tires and don’t see anything close to the tires that could be hitting them. It sounds like a strip of rubber hitting a metal pan. I see no loose plastic guards or mudflaps. The tires were all replaced less than 2 years ago and they have been rotated regularly and visually appear to be in good shape. If anyone has any suggestions, I would greatly appreciate it.
If they replaced brake parts, they probably had to hang the brake caliper temporarily out of the way, likely suspending it from a bungie cord or coat hanger. If they’ve forgot to remove that bungie or coat hanger, it could make quite a racket - ask me how I know. 
Remove the wheels and look up into the wheelwell for stuff that does not belong.
They most likely attached it to the spring or upper control arm.
Also, if they replaced the rotors, and didn’t clean the mating surfaces, there may be something small making a rotor not spin true, causing a slight rubbing, after time would heat up the pads, making them squeak or make noise. It would be tough to find this by just spinning, but if you have someone with you while you drive, and they stick their head out the window and look, they may be able to see a wheel spinning slightly off-whack.
You can also try rotating the tires to see if the flapping/knocking moves with a tire. Just because they’ve been rotated properly doesn’t mean there’s not something stuck in one, or one’s not quite as good as it seems.
Good luck,
Chase
Maybe “Grandma got run over by a Camry!” and she’s stuck under the car! . . . . sorry, but I had to say it. Seriously, my only suggestion after you had the original mechanic look over his/her work and after you visually inspected for anything loose, is to have another mechanic put the car up on a rack (that’s what we call it here in my locale) and look over things once again. Obviously something is hitting something or something is loose and causing some noise . . and I wouldn’t let it go (myself, that is) as noises tend to suggest problems which only get worse and more expensive and dangerous as they are ignored. I like RemcoWs idea except that it would make noise all the time, and you say it happens only after driving for 5-10 minutes, which to me sounds like something has to warm-up (exhaust hanger?) before it starts to make noise. It’s not that uncommon for an exhaust hanger to be bynpoed out of place just slightly when the car is on the rack for service and when it’s on the road it could be out of place just enough, when warmed-up, to allow a part of your exhaust to bump, or rattle with engine rpm. You said that bumps don’t affect it, so I’d let it warm up and then put it up on a rack, put it in drive and allow it to run a little while you’re searching for the noise. Sorry for the long answer. Good luck! Rocketman
This is a FWD car, so if (a big if, really) the noise is coming from the rear…letting it run on a rack won’t have an impact. I do like the idea of getting it nice and hot, though.
Chase