1. The noise was not there for the first week or two of running the furnace.
2. Maybe this is irrelevant but generally it has not been cold enough to worry about freezing.
As for the old house, not sure. It was a small 550 sq foot one bedroom apartment. For what it was worth, the unit was located in the bathroom ceiling. However, the furnace filter, which maintenance replaced one or twice a year no matter what, was located just outside the bathroom in what appeared to be a large heating vent the size of say a vent you would see in a drop ceiling in a business.This is not a house either. It is a rental property with 4 or 5 buildings. Each building contains four town house units.
Let me add that just as I type this it did the hissing noise for a few seconds. This time the furnace had been on for maybe 1 minute. I waited to see if it would shut off, but it didn't. Then a few minuets after this, it did it again for about 3 seconds. Then maybe 2 or 3 minuets after the second hiss it shut off.
The reason I have been so persistent is because of the alarming rate of issues I am having with property management. We had about a 5MM gap in between the top and bottom window in our living room allowing wind and bugs in. It took a week to be "fixed" with weather stripping tape and duct tape on the inside and outside. Same for windows in our enclosed unfinished balcony/"storage area" that leaked "when the wind blows just right." One maintenance man has done a no call no show. We had no working smoke detectors in our unit, and I found that when I removed the upstairs smoke detector from the ceiling, I found a live exposed coper wire hanging loose. After the first maintenance man no called no showed, she claimed the second one had an "emergency" when he was scheduled to come out. And this one has me alarmed, we had a pizza delivered the other night. My girlfriend got the door for a change and the guy was telling us that he was almost our neighbor but found a ton of mold in the unit and the property manager claimed they would not do anything about it. Oh and the maintenance supervisor(?), went out of town, left no one capable of fulfilling his duties, and gave an estimated return date of late one week or early the next. If you have taken the time to read this slightly incoherent quick rant, you can understand why I am not taking the property manager's word that there is nothing wrong.