metallic noise while braking in 4wd

hi guys, need a little help
so, a while back i parked the car on a hill with 4wd engaged, went of to work, got back, took it out of 4wd and drove away, after a few miles, under hard braking, the car made a scary noise, a metallic grinding/banging, like a shaft spinning out of axis and hitting someting close to it. Seemed to come from the front left wheel. I stopped the car, had a look under it and nothing...
drove home just fine, didn't happen again.
After a few days i had the pads and rotors replaced as they needed some care. At the time i mentioned it to my mechanic, told him to keep an eye out for anything, but he didn't notice a thing.
Was fine for months, but then again, i din't use 4wd so...
This last week i went on vacation, got myself in the woods, and into 4wd. As soon as i used the brakes a little bit harder (but not locking the wheels), the noise came back.
The first time it happened i put it down as some glitch in the locking hubs...
But this time i was able to make sure it only happens while in 4wd, high or low...
Like i said, sounds like a shaft spining out of axis that's spinning and grinding on something.
Now, i have a problem in my clutch, it burns while under load, like doing over 60mph and starting to go over an incline, so i'm going for a new clutch. Could this be related? since this noise only happens in 4wd and at the front, and the car has some vibrations at high speed, i'm thinking maybe it's down to a failed box or engine mount...
Also, i don't feel anything when this happens, not on the steering, not on the pedals...just a slight vibration coming from the front..
Anyone has any ideas?
Thanks in advance

I'm not sure what it could be but there a few things you can do to see if there are any more clues.

Get under the car and check the front prop doesn't have any play in the joints.
Engage 4x4 and jack up the front wheels one at a time. Try to turn them and see if you can hear or feel anything strange from the hubs or the diff. Then try again with both front wheels jacked up. And again with it in 2wd.
Check for play in the wheel bearing and kingpins.

Check the hubs are enganging- with 4wd selected, when you turn one front wheel the other should turn the opposite way.

I doubt it is linked to the clutch issue.

thanks.
well, the rear prop has some play, but not on the u-joints, it's either in the box or the rear diff, almost a third of a turn. other thant that, no play in any of the props.
gonna do some checks this weekend before sending it to the shop next week.
the reasoning behind my guess has to do with the cluch burning and the vibrations. i'm assuming there is some play in the transfer box mounts, which when slowing down in gear and droping the rpms while braking would increase the vibrations and make this noise. but it does sound like something spining out of axis, not just banging. but yeah, that being related to the cluch burning is probably a stretch. but it's wierd, it burns at high speed, and it burns in traffic jams, but not when i do fast and sudden accelerations, i can floor it out of lights, and nothing, but will stuck in a traffic jam, or climbing a inclin over 60mps it smells like burning.
just hope it's not stupid expensive

That sounds strange you should smell the clutch at acceleration , it sounds like something is jamming at low speeds. may be the brakes are coming on, when it happens have you checked the front hubs and brakes are getting hot.