The a/c clutch should not stay engaged all the time when the a/c switch is turned on. The system is designed to shut off the clutch if the pressure is too hi or too low or the evaporator freezes up. This is to save the pump from damage. The noise you hear is the hi and low side if the system equalizing. To find a leak I have had the best luck recovering the remaining freon and charging the system with nitrogen to 50 and 275 psi and spraying the hoses, condensor, recevier drier, tube conections, shrader valves, relief valve, compressor with soapy water and looking for bubbles. I have used sniffers, injected dye in the systen and used a black light. But best of luck personally with nitrogen. I check at high and low pressure because I have had o -rings seep at low pressure but not high as well at hoses at crimps. If you don't see a leak checking items above, pull the clutch and see if it's leaking at thd shaft seal. If no luck it could be the evaporator so you would have to pull the dash and split the box and then you may see pag oil at the location of the leak. Any time you replace the compressor, replace the reciever dryer. If the compressor failed and made metal (rotary group broke) you need to flush the system and replace the oriface or expansion valve. Yes a/c can take a bunch of special tools and is labor intensive. Once you find the leak, you need to evac the system whick pulls to moiture out if the system and charge by weight. Moisture combines with freon and can form an acidic solution and will attack your componets. There is a bunch of good info in the support section at sandendotcom. I don't believe in any of the conditioners or stop leak "snake oils" added to some refrigerants as band aids. Hope this helps.
Charles