Hi,

I have a 1999 F150 5.4L 4X4. My OD light blinks only in Drive and sometimes reverse.

The OD Light only blinks after it's been in park for a minute. Then when I shift down to reverse it's still blinking. but when in netural, drive, 2, and 1 it stops. Also when I put it in reserve after going through all the gears reverse does not blink anymore.

My check engine light is on. It turns off only if I don't leave it in park too long. but as soon as I have it in park for a while the check engine light will turn back on.

There is also a hissing noise from the transmission only when the OD light is blinking. sounds like a belt but I don't think it is cause it's coming from the transmission. any help would be appreciated.

 

Sounds like a broken vacuum line or MLPS. How about running by autozone and having them pull codes. That'll give you a place to start. The computer is seeing something, that's why you get the lights.