A/C question

What is your question?

My heater hadn't worked so I replaced my t-stat but noticed no coolant in reservoir but had no light or gauge showing overheating? couldn't find a leak anywhere either so a friend/mechanic told me to replace pressure cap on reservoir, wha-la fixed the loosing antifreeze part and had heat for a while. Now my COLD air starts out cold then goes warm with a stale smell inside truck, sometimes goes back cold after 15-25 mins. I added Freon and helped for a couple hours but then started warming again with smell. Just this week I've noticed a whistle sound (like a kid learning how to whistle) very short toot, while truck is moving, either rolling or pressing gas pedal/brake. Could it be a leak in the dash? Or under hood?