i realize that you have a question, and asked it here, it’s better to ask, then to either assume, or make mistakes at the DMV test site.
however, if you are in any CDL school right now, that question would have been better asked to your instructor(s). in fact, your instructor should have told you it’s an “air system”, which would mean, no fluids of any kind in there.
on an outside “chance” your truck has a bad air-compressor, there “might be” a trace of oil seeping out, but in all my years, i have never seen that…
so ” NO audible leak” would be all that you’d be saying…