You didnt say if you added any mods recently or not. Just adding a K&N airfilter will cause really bad cold engine bogs, and sometimes sporadic rough idles when cold. Easy fix to that problem, but I'm not going to go into it unless you added something.
As for you hissing sound, all the 4.6 Stangs I've heard, make a simialr sound, I havent figured out yet what it is, but I havent looked to closely or hard, Alls I know is that it comes from the intake area.
Hmmmmm.....check for a vacumm leak first. Sounds to me like your problems are related to one another. Sorry, I know that sounds like lame advice, I just got home from a 12 hour day at the office and my brains fried.
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64 1/2 "D" code Red Mustang Coupe. 289, C4, 3:1 rear gear. Mallory duel point. Ported & Polished 65 heads shaved .01 with 351 windsor valves, high side of 10.5:1 comp, 1.7:1 sled rockers, blue wolverine lumpy cam, autolite 4100 Hipo 4 barrel. And to many others to list
2000 Perf Red Mustang GT. 5spd. BBK Underdrive pulleys, Flotech off-road H pipe. Hurst T-Handle
64 1/2 red 6cyl coupe. Auto. project car.
[This message has been edited by Mercury (edited 05-01-2001).]