What is a 'power supply noise filter'?

Surge Suppressors are commonly available as consumer units.

Power Supply Noise Filters are not particularly common as a consumer unit, though they are commonly available as parts to be built into commercial electronics.

You could simply use a better-built 9V DC supply which has a filter built in - it's fairly common to have one large-amperage 9V DC supply powering your whole pedalboard (if you have a few pedals), rather than individual wall-warts for each pedal. Those will normally have excellent filters.

For test purposes (it's expensive beyond that) you can also use a 9V battery to completely eliminate the possibility that power supply noise is a problem.

As to the question of "what a power supply noise filter is" it's usually 3 capacitors and 2 inductors in a package. You'll find better results searching for "EMI filter" usually. But I seriously doubt you need to go buy one, if you can even find a "plug and play" packaged for consumers version.

I'd use an Isobar

While dedicated (standalone) mains filters are generally not available as consumer products, the good news is that Tripp-Lite still makes their Isobar line of combination surge suppressors and AC mains filters. A two-outlet, plug-in Isoblok costs about $30 at the time of this writing, and is manufacturer specified to provide at least 40dB of mains noise suppression, which should be sufficient to keep your guitar pedal for emitting noises. Of course, using a wall-wart that's of a better grade than the cheapest thing that will pass FCC and UL testing (you could even use a medical grade supply, but that'd be overkill) will help with this, too.