We probably need to know the entire electrical signal chain, starting with the DP and going through to the speakers plus anything else on the circuit. These sorts of issues can be quite strange to debug. I've had faulty wiring cause it once: an exhaust fan was on a shared circuit with a poorly bonded neutral, and every time the fan came on or off, it sent a small overvolt into the speakers causing crackling and even caused the USB on the computer to reset. Sometimes it's the interface, sometimes it's the speaker amplifier, sometimes it's the speaker drivers, sometimes it's the cable. The list goes on... My most notorious hum puzzle came from a new GPU causing overvolts and undervolts in the PCI-e power draw, which an internal sound card translated into noise. It can be dozens of things. Some people will recommend that you can only get perfect results with either a dedicated subpanel or an exclusive utility hookup for a sound studio, but I've found that to be overkill/unnecessary for any small studio--something where you have a 32+ channel mixer, giant near-field monitors, and several isolated rooms is its own animal. Basically, if all of your equipment can run off of one 15-20amp residential breaker, that advice won't apply.
Usually grounding doesn't do much in the grand scheme of things (it's important and shouldn't be interfered with but is becoming decreasingly necessary), especially on more modern equipment, hence most big appliances now are simply two-pronged. Grounding really matters most when there's a mass of conductive metal in something (read KitchenAid stand mixer or three-phase tablesaw), and then only when the unit is experiencing a lightning strike or major electrical fault.
If you're house has only has a 60-70 year old fuse box, then you're wiring likely has deeper problems than just this. (If you've got knob and tube, I'm proud to know ya' and really can't help you!) Supposedly putting a GFCI breaker or outlet at the start of the branch circuit will give you essentially the same functionality as a grounding pin--though it won't ground, it will trip. There are some other ways to add grounding--including obviously adding a ground wire, but I don't know if they're really going to change much about the sounds you're getting. Usually noises are related to undervolts and overvolts on a shared branch circuit (look for any kind of a rogue appliance or something on the same circuit--sometimes even within your computer), and only low-freq mains hum (50/100/200hz in the UK and 60/120/240hz in the US) by the hum ground isolators. Using a voltage meter (safely and correctly) or hiring someone to debug this, might show some faults with the outlet or with the entire electrical supply to your house (every local utility has their own dialect of power issues at different times of day as the grid is a giant living organism that requires constant care), and there are a few other ways and tools to look at the situation.
As always, if you're working with electricity directly be extremely careful--no matter how much experience you have, and if you're at all uncomfortable or lacking knowledge/experience, hire a professional--without question. The risks are real and the cost of an electrician is nothing compared to any consequences of things going wrong! At the end of the day, I'd rather see someone have a host of annoying computer sound issues than any risk of injury!
Spotify:
https://open.spotify.com/artist/2xHiPcCsm29R12HX4eXd4JPianoteq Studio & Organteq
Casio GP300 & Custom organ console
