[tomj76]

If it's dye then it will wipe off. Clean it off, see if it comes back.

I think an A/C leak that is big enough to be heard is going to empty the system quickly. You might be hearing the flow of liquid through the expansion valve. I've read this is normal in some A/C systems.

I really don't know but I wonder if low charge in the system could lead to hearing this sound as the expansion valve attempts to compensate for the reduced available super-heat. I think the front a/c evaporator loses cooling more rapidly than the rear one when the charge is low.