weird click..help urgently needed?
 

Hello everyone,
I am in urgent need of help as i have a long drive coming up shortly.
The car is a 2000 grand marquis with 180k miles
This just started today...While stopped at a light, i had my car in "D" with the brakes applied. I heard a very faint clicking noise coming from the driver side or maybe it feels that way as i am the driver . (most of the time i shift to park at lights )
Later upon reaching home, i tried and was able to reproduce that click.
It only happens with the car being in "drive" and stopped with the brakes applied. Does not happen in neutral and does not happen in reverse with the brakes applied either. The car drives and shifts normally.
The clicking is so faint that i could only hear it inside the car. Not audible from outside. I am scared that it might be something related to the transmission.

Please, if anybody has any knowledge or experience on this issue, do let me know as i have a trip coming up. Do you think i can safely drive the car on a 600miler trip.??

Thank you so much