While I agree with Bob on all of the points he made, I would also take it a step back further.
1. What is the brand of the speakers?
2. When you say, “surround fronts,” are they your main front L/R speakers, or are they being used in addition to your front L/R mains for/as a, “Height Channel which some A/V receiver brands offer, (typically Yamaha).
If they are your dedicated front L/R channels speakers in a 5.1 or 7.1 system, you more than likely have a defective driver in both. Most people believe that a speaker is blown when it no longer works/operates. This is simply not true. A driver may be, “stressed,” whereas is still works, but will distort at a specific frequency and/or volume. Thus, the distortion you’re hearing is intermittent. Very common….
If they are additional, “Height Channels,” to your L/R mains, then there’s 2 ways to find out if it’s the speaker, ground loop, or the A/V receiver itself.
1. Connect the distorting speakers all by themselves into the main L/R outputs of your receiver. If they sound fine, then you know it’s not a problem with the speakers.
2. Take another pair of speakers, (as an example), your existing rears and hook them up to the same height channel outputs. Since you have no problem with your existing rear speakers, if they begin to distort on the same output, then you definitely have either an A/V receiver problem, or the other factors that Bob mentioned.
Best of luck….