Speakers make a buzzing noise but wont play anything else

Things to check:
1. With your multi meter, measure from left ground to right ground. What do you get? Measure from each input ground to chassis. What do you get there.

2. Get or make a pair of shorting plugs. Plug those into the inputs. Power on. Hum there or gone?

3. Turn the pre-amp on. Connect. Level of hum change with preamp on?

4 additional information. How efficient are your speakers? This is a big amp, high-efficiency speakers may show some noise, while low-efficiency speakers won't.

A trip back to the shop may be indicated, too.

Buzzing nois of this kind can come from a short or from disturbance.

your power supply 5volt is under powered with filtering issues.

Only the one channel buzzes? Did you check the cable of the speaker is it properly rolled?

I was using a 25 foot 1/4" to XLR cable from the back of the Roland directly into the Thump. As mentioned, I tried going from the Roland to a direct box with a 1/4 cable on both ends; then out from the direct box "thru"-1/4 fitting, to the XLR connection in the Mackie Thump, with a cable that had the 1/4 on one end and the XLR on the other. Still buzzing. So for grins, I hooked an XLR to XLR cable from the Direct Box Low-Z out, to the Mackie Thump, keeping the 1/4 plug going from the Roland to the Direct box and eliminating the "thru" connection from the direct box to the Thunp, (1/4 to xlr). NO BUZZ.
There is a definite loss of signal strength, ie: everything is coming through quieter, but I can raise the volume of the Thump to FULL, with no buzz, and I can raise the volume of the Roland Module to full, and still no buzz. But there is a definite loss of overall volume, I think because I am not using the direct box properly. Hmmmm. As I understand the Direct box, the IN and THRU are supposed to be used as I was, going from the Roland module to the IN, then from the THRU to the speaker. The Low-Z outs are supposed to carry a signal to a mixer, I suppose to use a FOH PA system, or for recording purposes. Still, I have no buzz. Now...........

In looking over the cords, I suddenly realized that the 25' 1/4" to XLR could be the problem. The 1/4" plug is STEREO! Not mono. Could this be the cause of my buzz? I'm thinking I have a stereo plug, so 2 wires are involved, yet I'm using it in a mono imput in the roland. Perhaps this is creating my ground loop?

After getting a 1/4" MONO to XLR adaptor, attach it to my XLR to XLR cord, go from the back of the Roland (1/4") to the speaker directly (XLR) and see if my buzz is still gone.
The problem now becomes going from the Direct box to the speaker, since it needs to be 1/4 mono from the box, and xlr male to the speaker. I cannot locate a cable like that! So, I guess the next best solution is to get an adaptor to convert one end of the xlr/xlr cable to a 1/4 mono plug. At that point I don't think the cable would still be considered a 'balanced' line though, since it goes from 3 wire at the xlr to 2 wire at the 1/4 ts plug, right? The only other solution is to continue running out of the direct box as I am, using the Low-z out xlr connection, so I can continue to use the xlr/xlr cable.
Also there was a button on the direct box, called a PAD, for -15dB, I assume that what was causing my signal attenuation. When I pushed it in and it released, I suddenly had my volume back.

The voice coil on the cube speakers must be scraping against the magnet.Or the speaker cone must be damaged causing it to flutter at low frequency where there will be large cone movement. Try removing the phono jack from your musical instrument/ guitar and touch the tip of the jack with your finger. You will hear a hum, now turn down the volume and listen to the speakers very closely and see if you can identify the flutter. All the same you will have to fix the speaker or replace it. Have you ever dropped the speakers, if so the speaker frame could have warped causing the scraping noise.

change your pre-amp ic sure it will work. or maybe cold solder from the board.

The first thing I would do is pull the stereo out of the dash and unplug it . If the buzzing stops the stereo is bad get a new stereo. Its not hard to remove the stereo. Right above the cup holder there is a small cover you need a very small screwdriver to remove it . Under that is a few screws take those out then grab the plastic cover that runs around the stereo and pull towards the back window it simply pops off (No screws) After you remove that cover you will see 4 phillips head screws that hold the stereo in remove them. Last slide out the stereo there will be 1 plug with alot of wires that is the power and speakers . Unplug it . The other round plug is the FM antenna

Sounds like feedback. Check your speaker connections at the speaker and the main unit.

be certain its not your cable providing the noise by being ungrounded .or interference from some other device connected or around tv.
disconnect everything except the cable.
turn off all dimmer lighting around tv. try now.
are you using a cable box, if so bypass ,plug cable directly into tv.
try sound again.
then remove cable and plug in dvd or vcr. and try audio now.
if it continues no matter,
its a warranty repair or just use external speakers ,cable boxes can control the audio to ext speakers.