Are you really sure it's the pressure relief valve? There's two, one mounted on the dryer and most trucks have a mod (MWO) to put another one right at the compressor outlet. They are set to blow at 150psi. The occasional air escape heard is likely the dryer cycling. The dryer under the truck near the spare tire purges itself every time the compressor governor unloads (shuts off) the compressor when the tank reaches 125 psi. When that happens, the excess air in the compressor lines is diverted through an alternate path in the dryer to vent the accumulated snot and moisture out of the dryer overboard. This is the pop and hiss you hear. You will also hear something akin to air brakes pressurizing when the air pressure drops to 90 psi and the compressor kicks in. That's when the dryer resets to dry mode and the lines start to fill up.
All of this function is controlled by the compressor unloader, a little doodad mounted on the side of the compressor and has, as I recall, 3 pipes attached to it that run towards the rear of the truck. One is the compressed air outlet to the dryer and wet tank, one is the dryer control and one is the pressure sense line to control the unloader.
A normal A3, once initially filling the tanks and reaching 125psi, will purge the dryer. The compressor/dryer should not cycle again sooner than 10 or 15 minutes at idle with no brakes applied or steering wheel movement. If yours cycles more rapidly, you have a significant air leak that should be looked into because it will cause problems with the dryer, which causes other issues. The really common air leakers are the steering regulator and steering drag link and just general loose plumbing fittings throughout the system. There are many posts on these culprits that can be found by searching. For most air system issues, the truck line pressure gauge in the dash and your ears are the primary diagnostic tools. Significant leaks can be heard and you should see no drop in gauge pressure when the engine is idling or off given with no brakes applied, etc.
Also, make sure you have properly serviced the dryer filter and desiccant cartridge. The leaky air systems really gum them up and in turn, pass lots of snot and moisture into the truck main air system. That will screw up the CTIS and other stuff, rust the drag link of the air assist steering, etc.
