Hey Heres What I've Gathered
Ok, If you have done that "performance improvement" where you take the coolant hose from the EACV "Electronic Air Control Valve" and run it strait to the head of the engine; basically eliminating the heater valve where coolant runs through on underneath the throttle body, than that would be my first guess on a rough idle. That mod can work but not for 4th gen preludes consistently.
If you haven't done that then the next thing to do is check the coolant level from the top of the radiator or check for coolant leaks especially the hoses running to and from the EACV.
Step three if those two were'nt infact the case the other thing that can cause that is your throttle cable and idle adjusting screw. I don't know where you take your car to get fixed but both of those need to be tuned to 100% if you want your Honda to run well. First check to see if you even have your idle adjusting screw; if thats missing then you have probably solved both the high rev'ing problem and the whistle sound. If its there go ahead and try turning it to the right slowly but put pressure on the screw aswell, by turning the screw to the right you are dropping your idle "left is increasing idle". If you have turned it too far to where it wont tighten quicly turn the screw left all the way around about two times that should be pretty close to stock. 2nd after accomplishing nothing there, go ahead and reset your EACV and idle:
1). start your car till it has warmed up to it's driving temperature.
2). pull the grey electric sensor plug from the EACV with your car running, your check engine light will pop up blah, blah, blah; go turn your car off now with your sensor plug still un-plugged.
3). no go to your under-hood fuse box and pull the clock/radio fuse should be a 10-amp. fuse; wait atleast 20 sec. more time is always better for esurance that it really reset the idle and EACV but 20 sec. min.
4). now plug back in the silver sensor plug to the EACV, then plug back in the clock/radio fuse; you might hear a air relief "pfff" sound but it's not real loud so you may not.
5). after you have plugged back in both the fuse and sensor, start the car back up.
6). check your idle once the car is at normal temperature, your idle should be anywhere from 600-1250 rpms. at this point.
7). if your idle is reading high adjust the idle adjusting screw to the right till it reads somewhere around 750-900; if idle reads low turn idle adjusting screw to the left till it reads 750-900.
8). always adjust this screw slowly and check your rpms. all the time in-between.
9). do a quick test drive down the block or something, then stop and check your tachometer to make sure your idle is still holding where you had it. If it's not get out and adjust it, now your done with that procedure.

So say none of that has helped yet?!, now its time to adjust your throttle cable, this is one of those things that is freakin' irritating because you will never get it right the first time maybe not even the 15th time.
1). un-hook the cable from the two hold downs that just keep the cable from shaking and moving when your driving (one is connected to the top of the intake manifold, the other is close to the firewall.
2). now loosen the two nuts that are holding the throttle cable in position, now the cable can be moved left or right.
3). I would say to get close start the right nut at about the 5-7th thread on the cable, after that make sure the cable doesnt turn or move or bend when you go to tighten the left nut so that the cable is locked into position "best way is to take either two 12mm. wrenches and hold the right nut while tightening the left nut, or take some pliers and hold them very tight right behind the right nut but on the cable, hold the pliers snug and steady while you tighten the left nut tightly.
4). If you noticed your idle has stopped hi-revving then you know this is your problem but your not done yet.
5). now go for a drive and make sure your driving needs are fullfilled (power, sound, etc.)
6). adjust this cable the same way you did up-top, by like a 1/4 of a turn either way you decide. every time you do this adjusting step till you are satisfied and trust me, as a hardcore tuner especially on my lude this will take some time; but this is what keeps my all motor lude competing with the corvettes out there, by finding my ludes happy spot.

Now if for some reason your car still is working I would probably be freakin' pissed with all the work and time i've already spent but check your powersteering belt, make sure its adjusted correctly and tight to the mount, push on the belt and if theres some give, re-adjust the belt so that its tight to the pulleys, after tightening the three bolts to the mount now adjust the adjusting bolt "that bolt that sticks up from the power steering pump" tighten that snug with a 10mm. wratchet, then take a 12mm. wrench and tighten the lock-nut really snug to the surface this will keep the pump from possibly moving when driving and hitting bumps and will keep the belt evenly laid on the pulleys.

Ok if none of this worked for you man post another message saying thanks but no thanks and i will try to get a better idea as to what exactly your car is doing but I had the problems your having and this is what I did to get rid of it so i do believe you wont be spending much if anymore money to fix this annoying problem, oh and the hsss! noise you hear could also be coming from your brake reservoir cap those were made cheaply just get one of those cotton wristband things that jocks wear and put it around the top where the cap locks to the reservoir and that will work.

Sorry there was a lot to read but it could be worth it to read! later and goodluck man!