due to trade my q7 in soon and have just noticed a hissing sound coming from underneath the left side footwell, like an air leak. Lasts around 15 - 20 min after the car is turned off

the suspension is not dropping and all works as it should, i have set it at off road at the moment and its seems to be holding.

it is due in at my friends garage next week to replace the actuator on the turbo but he hasn't had time to have a look at this problem yet, i was hoping if it needed a part to get it and have them done at the same time.

i popped into another garage today and they put it up on the ramps, without taking the plastics off it sounds like it is coming from the big aluminum tank under the passenger side, i could see some writing saying hydro on it.

the lad seems to think that is the air tank for the suspension but not sure where the leak could be coming from, has anyone else come across this before, any suggestions

is there anything that runs off that tank that could be leaking air

thanks in advance