My truck had a sucking sound that changed pitch with throttle application. Sounded loudest at mid throttle. Could barely be heard at low or high throttle. I tried a new air cleaner gasket, cleaned tb/iac, new tb gasket. Did not help. My ac blew out the vents occassionally. Maybe once or twice a week, and I drive about 300 miles a week. I pulled the check valve by the intake manifold, and it worked fine blowing through it. Could blow one way, but not the other. I tried a couple more times, and noticed it would stick and not allow vacuum to flow the direction its supposed to. I replaced it, and noticed the sucking sound has greatly reduced and is hardly noticeable. Sounds like a normal engine now. Nice surprise, as I had given up on finding the noise and lived with it so long I started to think maybe it was normal and had always been there. Anyway, I've seen posts in the past where others had complained of the noise and maybe this can help someone. Only cost me about 5 bucks, so I'm diggin it.

 

PCV on the valvecover or the brake booster check valve? THat was not my sucking problem (was the way i had my 14x3 on there) but I am guessing it was not your brake booster valve. Ussually people call that the "check valve"

 

The check valve I changed is one at the intake manifold. There's a smaller vacuum port right below the vacuum port where the pcv valve hose attaches to the intake manifold. You'll find a rubber elbow on it, and the check valve (one way valve) is attached to the elbow. There's another just like it closer to the firewall. I have a stock air cleaner. Had a spectre filter, but I work in an extremely dusty environment and am afraid the cloth filter probably let's more dust through. I miss the filter, though. Checked mileage with the cloth filter several times and I always got around .5 mpg better (13 vs 12.5 mpg). Not a big difference, but every little bit helps on the old 5-9.