This was a common problem with early '70’s VW’s, and, as you guys said, is just from loose heads. A good tech could hear this problem on a bug/bus coming into the lot, since compression was also leaking past the head gasket.
However, loose heads in Vdubs were from the head/cylinder bolts actually pulling out of poor case castings, and tightening them will do nothing but worsen the problem. The answer was case inserts, with bigger and coarser threads, that the headbolts were then threaded into. Unfortunately, since you had to drill and tap the case, a complete takedown was in order.
We used to do this for $450. Of course, if we found bad rings, bearings, or anything else, it could go up from there 
HolySmokes