That I know of so far:
One rare in Waterworks explorer area
One rare in Tangleroot explorer area
5-8 in Gwylan's Stand in the first cave (I hate that place)
One rare in Redfang
At least 2 others that I don't remember which quest they were in.
There's still a ton of quests I've not done yet, so there may be more. They also seem to be coded specifically to aggro on warforged (which makes sense as you're basically walking lunch). I have stood out of the way back behind everyone purposefully doing absolutely nothing and had a rust monster pass straight through the rest of the party beating on it to come charging for me. We hates them, Precious, we hates them.
Amusingly, my warforged also seems to be a magnet for poison and disease immunity items in chests. I pass them on to my fleshy alts.
If you see a beholder, stand back and let the warforged kill it. If you see a rust monster, stand back and watch the warforged run.
