My ABS light comes on for approx 30/45 seconds after the engine is started

I am 6 days out of warranty, and my ABS light comes on for approx 30/45 seconds after the engine is started. It then goes out and comes on about 20 t0 30 miles later. When it comes on after driving awhile, it stays on untill I stop and restart the engine. The engine is the 6.0 Diesel if that makes any differance. This truck is the Super Duty with the built in trailor brake control. I called Ford and they said "sorry you are 6 days out of warranty". Thanks for any help, John.