Loud noise like popcorn or lose gravel being kicked up when furnace starts

I have a 5 year old gas furnace and A/C HVAC system.

It's a bit chilly tonight so my wife turned on the heater (first time in maybe a month) and it started making this very loud popcorn like sound - you can clearly hear something rattling around. It sounds like several things are being "kicked around." The noise is like gravel or bits of metal being blown around in a cylinder of sheet metal.

My initial thought was that there was some debris in the blower area and it was getting kicked around when the fan was running, but if we run just the fan there's no noise. The noise comes on when the furnace is starting (before the fan has even kicked on).

Any ideas what is going on? For now we're just foregoing the furnace and plan on calling the company that installed our HVAC on Monday. But am curious if anyone knows what might be the cause or what sort of damage we're looking at.

Thank you

submitted by /u/untaken-username
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