2001 F150.
Manual locks & windows on doors.
6cyl.
If keys were left in ignition while door was open, was slow rhythmic 'beep' or 'ping.'
Clifford security system was pulled out.
Thereafter, pinging noise:
1) would come on every time driver side door open, whether or not keys were in ignition; &
2) this pinging noise was very fast, much faster than previously.
We are stumped.