Sounds like the slip rings for the horn and air bag are squeeking.
Pre-air bag, the fix was to slightly lube the rings, post-air bag the fix is to just ignore the sound. According to the manuals DO NOT lube the slip rings.
Aletrnate repair would be to replace the slips rings. Parts are about $75.00, at least for a 126, not sure about newer models.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.