Very strange noise when braking

First time user here…looking for some help.
Four months ago a strange sqeak and a flap, flap noise (like a valve opening and shutting) coming from the rear of my vehicle when applying the brakes!
I just had all four rotors and brakes changed, I check to see if there was something loose or rattling around in vehicle and check the exterior of vehicle for loose mud flaps etc. I can not for the life of me figure this one out! Please, please help!!

Not sure what do you mean by flapping noise, but if it rattles I’d check if the shop put brake pads incorrectly (yup, it’s possible). If it squeaks, then they most likely were not lubed properly during the install.

It likely is not the brakes. The brakes are in the equation ans they apply force to the suspension parts that in tern, apply forces to the body and the tires. Take it in to a trusted independent mechanic and tell they what you told us and more. As much as you can. Does it happen all the time? Only sometimes? When hot or cold? When driving short trips or long? Hard braking, soft braking?

The mechanic should put the car on a lift and give it a thorough look-see. Expect to pay for his time.

If he finds nothing, have him rotate the tires front to back. As you drive away, check the brakes and see if the noise moved to the front. If so, return and have the tires thoroughly checked and maybe replaced.

No rattling. Like a screen door shutting but doesn’t close the first time. Flap, flap.

My tires are new. It’s doesn’t happen every time but 99% of the time. It happens in all weather. Happens on long trips and short. But I’ll get them to look. Thank you!

Make sure to check the heat shield. Could be banging on the muffler if loose.

Check the tires anyway, even if new. It could still be a tire problem. Good Luck! Post back with what you find, if you please.

Thanks all! I will bring it in and let you know.