What causes the return linset to constantly run hot, so much that you cannot touch it for more than a few seconds? A/C repairman says everything checks out okay with the system. It also struggles to maintain temp on hotter days. Not just running more than usual, will run for several hours at a time without shutting off. I can also hear the linset whissling through the walls on it's way up to the blower in the attic. My other A/C on the 1st floor (two zone, same model) does not behave the same way. Any input would be appreciated. Thanks

Dirty outside condenser unit will cause high pressures and a hot/warm liquid line.

If this is a heat pump the reversing valve could be partially open.

Dirty indoor coil/filter.

You need to find a more qualified A/C repair person.