Could someone please help to locate the air filter for my HVAC system? Attached is a picture of the heater with the front panel removed: [url]https://imgur.com/a/ihIFw9z[/url] The panel below the removed one doesn't come off so I didn't try to force it. I have tried to under behind the vents below the unit, but there is nothing there.

What’s up I just moved into a new apartment about 2 days ago and we have an old natural gas heater... Siegler model # 775 UB -2-S.... when we first moved in the property owner told me that the heater worked but was very loud and that he was going to have it fixed ASAP; I’m a certified General Motors technician and I like to consider myself pretty handy so I figured I’d look into it myself; the heater would work fine and blow hot air but was very very noisey and would shake pretty bad; the first thing I did was removed the back panel including the blower motor and fan assembly and found that there was probably thirty five years worth of dust and crap built up everywhere including on the fan; I took apart the fan and motor assembly and I cleaned everything to the point that I was satisfied and put it back together; when reassembled the shaking stopped so I’m assuming that there was so much buildup on the fan that it was just very unbalanced but I still had a bad rattle noise; I removed the front grill and glass panels and replaced the rope gasket and installed new hardware to make sure that the glass was tight and also cleaned out the front and anything that I could reach; when I put it back together it worked perfectly and was quiet and smooth as probably the day it was manufactured; however when it is on for a while I get an intermittent rattle from somewhere in the front of the unit; I have verified that it is not the glass or anything on that front grill; I’ve been trying to find a manual for this thing and can’t anywhere; any help would be greatly appreciated