I've had this happen once a few years ago with a packaged unit as well. I was told it was a loose piece of insulation at the elbow duct that would cover the supply side occasionally which then forced the air back through the returm. Don't know that I completely believe it. Then again I didn't believe it when the air came out the return and the after letting the unit sit for about a half hour it came out the supply registers when I went to show the buyer and their agent. Luckily the seller was there to confirm the first as everyone thought I may had been drinking at lunch.