Why is my portable AC still struggling after I tried the dual-hose mod?

Those old storm windows are almost certainly your bottleneck right now. If that glass is still down, you've essentially created a high-pressure heat trap between the unit's exhaust and the outside world. The hot air hits the glass, bounces back, and creates a pocket of 120+ degree air that the exhaust fan can't push against. This causes **high head pressure** in the compressor, wich is why you're hearing that heavy vibration. The compressor is literally straining to move refrigerant because it can't shed the heat. You need to slide that storm window glass all the way up so the exhaust is blowing directly through the screen. You might even want to remove the screen entirely if the mesh is too fine, as those old metal screens can restrict airflow more than you'd think. Once that heat has a clear path out, the vibration shoud settle down as the compressor reaches a normal operating temperature. If it stays restricted, you risk frying the compressor or tripping the thermal overload switch.

Regarding the intake, make sure you aren't "short-cycling" the air. If the intake hose is pulling air from right next to where the exhaust is dumping out, you're just sucking hot air back into the condenser. Try to separate the two ports as much as possible on your window seal. Also, check that you used **foil tape** instead of standard duct tape. Duct tape adhesive tends to fail and get gooey under the high heat of an exhaust hose, which leads to leaks you might not notice at first glance. If the unit is still sitting at 79 degrees after fixing the window, check the internal coils. Sometimes during these DIY mods, the restricted airflow causes a bit of frost buildup on the evaporator. Give it an hour to run with the storm window open before you worry—it takes a while for these portable units to pull the thermal mass of a room down once the walls and furniture have soaked up all that heat.

Are you sure that crate is as solid as you think? Sometimes those DIY platforms act like a drum and amplify the compressor's natural hum. Have you ruled out the crate itself vibrating against the floor or the unit?

Also, I'm curious about the hose diameter you used for the intake. If it's too narrow or has too many bends, you might be hitting a static pressure limit. These portable fans are usually pretty weak; have you checked if the unit still vibrates the same way if you temporarily disconnect just the intake hose? If the noise drops, your fan is likely struggling against the air resistance you added.

Check the temperture of that intake hose. If you used thin ducting without real insulation, it's likely radiating heat right back into the room. Back in the day, we’d wrap lines in thick fiberglass batting to keep the heat where it belongs.

Also, don't trust that reflective stuff alone. If it's leaking air around the edges, it’s a waste of time. Grab a candle and move it slowly around your window seals. If the flame flickers, you’ve got a leak. Even a tiny gap will kill your cooling capacity. I’ve seen "perfect" DIY jobs fail becasue of a quarter-inch gap hidden behind the trim.

I had that exact rattle. For me, it was the stiff ducting pulling on the unit's plastic shell and making it buzz against the internal frame. I put a thick rubber gym mat under the unit and cinched the whole thing to the crate with a bungee cord. Silenced it immediatly. About the cooling—check your intake shroud. I originally taped my hose directly over the intake grate, and it choked the airflow. I had to build a "spacer" box out of foam board so the air coud spread across the whole coil before hitting the hose. Once I gave it that extra breathing room, the vent temps dropped 10 degrees.