I was not totally sure about about a leak inside the car. I closed up the windows overnight and left the electronic leak detector in the car. the following day I quickly opened the door and turned on the detector. It immediately alarmed. This lead me to the TX valve. It was a small leak at the valve. I hope you have a high quality detector. Searching for a leak in engine compartment without one is futile. I know this sounds like a backyard way of finding a leak but it worked. Good Luck.