I have beeps that sound whenever I exceed 90 mph. Since my car is gutted for race use only, for the last year I thought it was coming from my onboard lap timing system, but quickly figured out that it was not. I have learned to use it to my advantage... Without looking down at the speedo, I can gauge my speeds audibly while approaching corners...

Turns out to be a handy little (unexpected) driving aid... I don't know why yours activates at 70 mph...