Yeah, mine makes different sounds too, but while she works properly otherwise Im in no hurry to do anything.
Typically after I have descaled her she makes different sounds for a while.
Could also be a pipe connection to the pump that is very close to an internal panel, and when it touches it could make the type of sound you describe.
I think I have heard of someone who moved the pump, or insulated the mounts to make it quieter.
Could be a slightly kinked inlet pipe to the pump which slightly restricts the flow, relatively easy to follow the pipe from water reservoir to the pump.
Another possibility, could be related to boiler level when you start to pour.
Silvia was/is notorious for slight leak from the steam wand when you turn her on to warm up, (retension of the steam knob when you turn her on will fix the leak), if there has been a slight leak out the steam wand while she is warming up the first water into the boiler will be resetting the level and at lower pressure instead of producing the normal brew pressure, but I wouldnt expect the sound you describe if this was happening.
Finally, I have heard of leftover swarf being found in the boiler, which would intermittently affect any water outflows, however cant remember if it sounded different or just didnt flow very well, and this is relatively rare.