Grounding dc project to clean RF noise
Firstly, Looking at your photo, you need short cables, and on the main power cables, twist them with their respective 0v/earths, for good practice, even on a prototype. 

For a test see if you can get your hands on a ferrite core that attenuates at your switching power supply's frequency around 125kHz. Try robbing one from an old switching power supply (smpsu) . They look like a donut shape bulge on the output or input to a SMPSU. 

Place ferrites over all the output/or/ and input power supply cables,  including gnd, from the switch mode, then see if this helps. 

If this does help then it's common mode interference coming from the switching power supply. 

You can also try grabbing all the cables in your hand,  making sure they are insulated and not electrically connected,  and try and earth yourself.  It sounds silly but you are a capacitor to gnd for high frequency common mode signals! This will tell you it's common mode. 

But mainly in response to your question,  I would strongly recommend grounding to metal pipework, It's probably safer, and legal, to earth to your pipework, rather than your house's electrical system .  All mettle pipework should be earthed. 

You need to find the main path of interference,  these things can be very tricky to sort out and one can go round in circles,  but try the earthing to pipework,  with short multi core cable,  first.