Compressor noise

Last night Trane 16xli installed 2015 outside unit was making unusual noise, louder then usual, 18 deg f . went outside to check . Fan was running no apparent frost, both set lines were cool as was set lines inside at air handler . discharge air in house was approx 74. I decided to turn Tstat to emergency heat . Compressor turned off and aux provided heat . 15 min later I turned t stat back to heat setting and all seemed fine for the rest of the night . Pressures was checked in fall at less than optimal load in cool, subcool showed low sub cool number bit tech didnt want to possibly overcharge unit by chance . House did maintain set temp at that time but I did not like this much louder sound and lines feeling cool. all apperas ok right now . My concern is low refer charge . would you do superheat check ? I believe this unit should be accurately checked in the cooling subcool mode I dont want to overcharge or under .Thanks

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