The maximum negative pressure of LEAKSHIELD is mostly not a problem for a radiator.

As you heard a "ping" sound" I assume that he soldering of some fins to the brass tubes has been broken. This might happen at fins that are only slightly soldered to the tube. I'm sure this has no effect at the thermal performance of the radiator.
We haven't seen any radiator with an issue due the negative pressure of LEAKSHIELD. At the pressure test of an radiator you often also hear theses sounds due broken fin solderings. The material is very thin at watercooling radiators and any pressure change deforms it.
At many radiators some fins are not perfectly soldered and only hold in place by the paint. The paint can also break if there is any movement.

Of course this will not be possible at airplex modularity or radical radiators as we don't use soldered conenctions.