My home outside unit started making a very low humming noise, just prior to the compressor coming on, and after the compressor shuts off. This noise is barely audible, but continues for quite a while. I can kill the power to the unit and it still hums. The only thing that stops the hum is to turn the thermostat off of "cool" - either in the off or heat positions. The a/c works fine, but the humming noise is troublesome. I cannot tell exactly where the noise is coming from. I cannot hear it from the electronics in the unit, or near the solenoid or the compressor. I have replaced both the contactor and the capacitor with no impact on the hum noise, so I replaced the original ones. Thanks in advance for any ideas...

Trane Model 4WCC4042A1000AA (s/n 154113026L) self contained package A/C Service provider billed for 4 lbs. of R410a to correct a poor cooling issues. I was charged a reasonable service call fee plus parts being the Freon at 240.00 (60.00 per lb.). For a package 3.5 ton self contained unit, 4 lbs seems a little high. If that is correct, then it seems a serious leak exists. There was no leak test indicated on the bill. Second issue is with the price. I don’t have a license to purchase R410a, but the prices are available. I found a range between 89.00 to 145.00 for a 25lb. Cylinder (5.60 per lb.). That is a markup of 970% using the 145.00 cylinder price. Does the quantity of 4 lbs seem reasonable? We just bought the house last December and don’t have a history of the last service, but I assume if there was recent service it was more than a year ago. Should there have been a leak test? As for the Freon price, I don’t object paying a reasonable makeup for the expertise, but 970% seems excessive. Is such a high markup for Freon customary?