Flashing ABS light when beeping the horn or when turning the A/C on
 

Hi everybody!
I have a 2002 Chevy Express 3500 minibus with the 6.5 diesel and the ABS light lights up every time I beep the horn or turn the A/C on.
I cleaned all the grounds that I could find but no luck. I also had the system checked with a scanner and found no codes.
Has anybody experience anything like this or something similar?
I will appreciate any input!!
Thanks:)