interesting problem. lots of unknowns. doesn't sound like the system could be performing very well. 40 and 175 with 2 sc sounds like a low freon condition, but i don't know the other conditions in the system. given the age, there's no telling the service history and possible errors. have any freon leaks been identified ? i would be inclined to follow your line of thinking and start with a fresh charge. there could be air in the system contributing to the noise. it will certainly affect final pressures and performance.
more than 2 psi pressure drop across a suction line filter is a problem in itself.
i would recover the refrigerant and take it to a reclaim center. use nitrogen to pressure test all points. verify you don't have leaks, then replace the liquid line filter. cut out the suction line filter and replace it with pipe. pull a deep vacuum. clean both coils. make sure it has a good belt on the blower. surely you have a txv metering device, so charge by sub-cooling, then verify proper superheat. with this reset of conditions, you have removed some variables and you can monitor the system for other issues. also, you will be able to start a fresh maintenance log for future reference.