It is a very bad thing to run a vehicle without coolant for more than a couple of minutes. I mean a couple. If your engine is hot when the collant disappers shut it down right then and there. The money you spend on a tow will be nothing compared to the rebuild. If you are not capable or equipped to rebuild your engine on your own you are looking at at least $1500.

The rod and crank bearings are very easy to daamge, and once damaged the only repair is to tear the engine down to a bare block, and replace them. As the bearings fail they will allow the connecting rods to move in abnormal oscillations, this is bad, this means cylinder wall damage which translates to machine work. If you are capable of rebuilding your engine on your own, you will spend more at the machine shop repairing your crank and block than the tow will cost, although if you are capable of rebuilding your own engine you probably know this already.

ETA: If you had a spun bearing it would be a consistant noise. IT is probably a bearing that has been damaged, but once the engine warms up the parts expand enough to restrict the undesired movement of the damaged part. It will not get better, it may not get worse for a while, but I would be looking for catastrophic failure soon.