When braking sound from the rear

Hello, I have to ask the question a bit simultaneously, since the vehicle is no longer mine. The new owner asks about a sound when braking. I tested a lap once, and have never heard it before. Anyway, you step on the brake, comes a relatively low tone from behind, first long, then a little shorter, always on-off-off. Also standing when you step on the brake. Lt. the owner is in the manual what of ABS test (I don’t have a manual to look there anymore). D But if I have never heard these sounds in my long omega time, I am worried about that. Can anyone tell me the meaning and consequences of these sounds? No warning lights, brakes at the front are completely new, not complained about at the back of the TÜV (Tüv = 4 weeks old). It is the omega as it stands in the signature. Thanks! Jörg