RB20 in a Silvia: RPM's Drop When Brake Applied

[dbiz]

i literally just had a 240 delivered to my house with an rb20, and it idles at 1.8k before warmup.. i was told thats normal?? lmao anyway when i apply the brake (clutch in or in neutral), i hear a loud ass leak-like noise, just sounds like air pouring out or some shit, and the rpm's just start to drop. previous owner told me to just tap the gas once or twice to keep it up. i havent really checked it out or tried to find out what it is yet. any ideas what it could be??