not good. Could be a collapsed or stuck lifter which happened when the heads were machined - the lifter core sits farther into the lifter body. Could also be a valve tapping a piston if the lifter is stuck in the extended position. Or it could be a nut or bolt that got dropped. I would try a compression test to see if all the Cyl are equal first. Also you can use a long extension or screwdriver as a stethoscope to zero in on where the sound is coming from. That will tell you a little about the problem.
Lots of machine shops are not very careful when they do a valve job to keep the valve stem height the same as before the job, even though they will tell you they did. If the stem height changes radically it can cause all sorts of issues when combined with milling the heads.