Hi and welcome!
I don't know what climate you are living in; if both guinea pigs are making the same sounds off and on, then there is a likelihood that it is rather environmental.
I know from members in the Southwest US that a dry desert climate can cause breathing difficulties, as can air conditioning. Have you tried whether higher air humidity can make a difference? Just see whether a bowl of steaming water, which you keep renewing can help ease the breathing. Dust and pollen can be another avenue to pursue. How old are your guinea pigs? Have you tried to work out whether the issue is mostly located in the nose, the throat or furteher down in the lungs?
Guinea pigs can't breathe through their mouths, so every small obstruction is very audible. It can be very frustrating to get to the bottom of off-on breathing issues, as I know myself. It took me two years to get to the bottom of my elderly Ffraid's off and on crackly breathing. I know from her previous owner that she'd had URI when younger, but she had the bad habit of holding her breath and breathe very cleanly whenever examined by a vet, so a number of visits were spectacularly unsuccessful. In the end, she was diagnosed with lung disease and narrowed airways by a very experienced guinea pig vet with a very good stethoscope. She lived to 7 years old and died from another cause, by the way.
Since we have members from all over the world, we find it very helpful if you please added your country, state/province or UK county to your details, so we can take your climate, vet access and available brands into consideration straight away. You can do this by clicking on your username on the top bar, then go to personal details and scroll down to location. Thank you!