Why is my furnace making a loud grinding sound when the blower is on?

The fact that the noise changes when you open the front panel is a big clue. I had something similar on my old furnace where the blower would shudder and rattle worse with the door on, and it turned out the cabinet was starved for air. With the panel sealed up, the blower pulls harder against the restricted return side and the wheel starts to cavitate and rub, which sounds exactly like metallic grinding. With the panel off, it gets enough air and quiets right down.

First thing to check is your filter. A clogged one does this, and so does a filter that's too restrictive for the slot. Pull it out and run the unit briefly with no filter at all. If the grinding mostly goes away, you've found your problem and it's just airflow starvation.

If it still does it with no filter, take the blower compartment door off and watch the blower wheel while it runs. A wheel that's caked with dust on one side goes out of balance and will shudder and sometimes scrape the housing as speed ramps up. That's a cleaning job, not a replacement.

Also, if yours is a high efficiency unit with pipes running outside, a partly blocked intake can make the whole thing act wierd even when the vents look fine. Leaves and even wasp nests get sucked into those. If the intake pipe near the furnace isn't glued at the top joint, you can pull it loose and peek inside for a screen full of junk.

If none of that pans out, then yeah, book a tech. A motor bearing on its way out can also grind under load, and that's not a DIY fix. But I'd bet on airflow or a dirty wheel before anything expensive.

I've seen this happen on some of the older units from back in the day. Sometimes the issue isn't the air, but how the machine is acutally bolted down. If those rubber pads underneath the furnace mounts have gone brittle or simply failed, the whole metal shell starts vibrating. That vibration bounces into the fan motor and sounds like a terrible metallic grind. Try tapping the unit lightly with a rubber mallet to see if the noise changes. If it does, you probly just need to tighten the mounting bolts or replace those old isolators.

Kind of related to what's happening here, but when my machine started howling last winter I figured out the metal door itself was the problem. It was slightly bent at the bottom and would flex when the fan spun up, so the whole thing acted like a drum. Pushing on the door with my hand made the noise change, wich is how I knew. I put a couple of strips of foam tape around the edge where it meets the cabinet and it calmed down alot. So maybe try pressing gently on the cover while it runs and see if the sound shifts? Also mine has a little button inside that cuts power when the door comes off, does yours do that too? Wondering if that matters for testing things.

One thing I'd add is the set screw that clamps the blower wheel to the motor shaft. On my old unit the shudder only showed up once the motor got up to speed, and it turned out the screw had loosened just enough that the wheel woud slip under load, wobble, and drag against the cut-off plate, then settle back down once everything cooled and stopped. The tell was that the wheel sat a hair further out on the shaft than the factory mark, which I only noticed after lining up the old scuff ring on the housing. With the blower door off, spin the wheel by hand and watch wheather it sits centered in the opening and turns freely without any side play. If it wobbles or the screw turns with light finger pressure, that's your culprit and it's a five minute fix with an Allen key once you've pulled the assembly.

Not sold on the advice to run the thing with no filter to test it. That's the one part of that plan I'd skip. Around here any dust the filter normally catches just ends up on the blower wheel or the coil instead, so you coud turn a dirty filter problem into a dirty wheel problem in one afternoon. Just hold the filter up to a light first. If you can't see light through it, that's your answer, no test run needed. Swap in a fresh one of the same size and cheap rating, don't jump to a fancy high filtration one, those choke some units too.