As said, you want to match or exceed as many parameters as you can, but it's rare that test equipment uses fans that are designed for sound reduction, so there is probably something available that will match size and airflow requirements that is at least a bit quieter.
Again, much of this has been said, but there are some other factors to consider:
If you have extra clearance for a thicker fan, going to a thicker one can mean lower RPM for the same airflow, so you can get a quieter rated replacement.
The turbulence generated by nearby parts or grills will often dominate fan noise - with a lot of manual labor you can try dulling edges on punched metal and such... but it's usually not worth it. You probably want the shielding to be in place when it's on test gear, but a rubber or nylon washer or two worth of thickness between the fan and the vent holes can actually reduce turbulence considerably.
If the design doesn't already have them, using rubber mounts (washers, O rings through the mounting holes, or full sized gaskets) can help reduce any vibration noise from the fan.
With high airflow fans in test equipment not designed to be really quiet, you really can't expect silence, but you can generally improve it considerably. If you happen to be looking for 120mm fans without the space for thicker than 25mm and you need high airflow (100ish CFM), I can recommend the Scythe Slipstream series. Fairly cheap compared to many quiet high airflow fans (<$15), but I swapped one into a recent repair and while it's far from silent, it is MUCH quieter than it was.