What's broke?

Looking for best guesses on new breakage.

It's a transmission issue. It might be clutch related, but kind'a doubt it.

Engine running, trans in neutral, clutch pedal pushed, things are quiet. Let the pedal out, and I can hear and feel (in the shifter) a gravelly racket. Not quite rocks in a blender, but there is certainly something crunchy going on in the milkshake. Not a 'thin' noise, but grumbly.

Trans shifts fine, motates fine. Nothing of particular note took place immediately prior to this. No whams, bams or anything unusual. The noise/vibration wasn't there one minute (parked in the garage), five minutes later it was (after a short run down the street to mix-up new gear lube in the diff).

I did have the impression there was a slight overall vibe when driving last time out that I was unsure was real or I was making it up. This engine has always been as smooth as glass, but the last time I took it for a spin, something didn't feel quite right. ...just a slight overall tremor that I maybe felt in the car. I don't know it to be related, or even pertinent, OR even a 'thing'.

I've always considered 833's to be unbreakable...behind a small block anyway. Only problem I ever had with one is a 2nd gear synchro failure when I was racing on a regular basis. This is NOT that tranny.

Any opines would be appreciated. Anyone with a known good 855 looking for a new home...I could find a nice spot for it.

Thanks!