I also own an 87 420 , these are great cars ,the hissing noise under the car is the 2 fuel pumps ,, that noise is normal , when the temp control thumbdial is all the way into the blue position you will feel the dial "click", that is the A/C clutch always engaged position , you should here the A/C clutch engage, if you don't ,you may need to add some R-12 if your system has not been changed to R-134, or you could have a CCU issue,or air duct control rubber boot issue, does your heat and defrost work accordingly? also it's very important to check the timing chain guides and tensioner , especially if you here a metalic chatter upon start up , that's the chain scraping metal because it has no tension or guide support. this is a sign that you don't want to ignore, it can lead to catastrophic engine failure..

Ben