The only time my sureflow mods make noise is if I shut them off, when I turn back on sometimes they will spin in the wrong direction, making a lot of noise. They either right themselves in a few seconds or I stick my finger in the noisy PH, stop the impeller, and it spins in the right direction, and no more noise. If it is making noise, it is usually from the impeller stopper, you can trim it back some if you must. Mine are just as quiet now as they were before I modded them. I did have one that took about a week or so before it ran silent like the two other ones I have. I can't speak of the Gulfstream, or the Eco brands. The mods do work best on the 900's-400's(not as noisy for some reason, I really only had one be noisy for about a week, then I guess it wore itself in, didn't do anything to it and now it is as quiet as the others, no buzzing or rattling sounds coming from them) from what I have read. I have 2 modded 1200's and 1 900 modded, all are quiet as can be now. It may just take a little tinkering to quiet them up if too noisy.
Matt