Temperature difference is too low again

For the last 3-4 years I keep having to call an AC company to check my heat pump because it's not cooling enough. This year no different. I have a 14 year old unit that today is showing a 6 degree delta temp between my return and the nearest register. Outside temp is 82. During the winter months, the outside unit makes a loud screeching sound about 3 minutes after it shuts off. Last year the AC guy said its just the reversing valve and said it should be fine. Could this be causing my issues with my cooling. I'm sure my delta temp 5 years ago was 19 degrees and this year it's 6 degrees. Surely it can't just be slightly low on refrigerant every single year. Thoughts?