Is HP 6623A Linear or Switching power supply?
It is definitely a linear power supply.  As described in the manual, it uses transistors as linear pass elements to regulate the output voltage. No switching elements are used. As far as noise is concerned, the manual specifies a value of 3 mv P-P for PARD (periodic and random deviation) over a 20 MHz bandwidth. If this is being exceeded, then there is a good chance that the filter capacitors are probably in need of replacement.