ChevyBunny – Welcome. I’m sorry your bunny is not feeling so well. The louder gurgling can indicate something is going on, and like Sarita said, the digestive system is the one alarm system that can go off for almost anything. I think many members here have dealt with bunny digestive issues (and if not, they most likely will at some point in their bunny’s life). I have had this happen due to different issues: Teeth problems, antiobiotics, diet – too many carbs/sweets, illness.
Right now you can try Simethicone just in case it’s gas and you want to help relieve pain. However, obviously something ongoing is causing this so this is not to replace a vet visit. Your bunny obviously has been dealing with this for awhile and so this is not THE fix but may offer temporary relief it his digestive system is creating gas.
Get baby formula – like “Little Tummies”. It’s also important to keep him hydrated, so give him wet greens to eat.
Here are the instructions for amount from Dana Krempels article: For relief of acute gas pain, 1-2 cc (20mg/ml suspension) can be given as often as every hour for three doses, then 1 cc every three to eight hours. This substance has no known drug interactions, is not absorbed through the intestinal lining and acts only on a mechanical principle: it changes the surface tension of the frothy gas bubbles in the gut, joining them into larger, easier-to-pass bubbles. is practically inert, and is safe to give, even as a precaution, as long as it is not given long term. (Note: liquid suspensions of simethicone are relatively expensive. Less expensive versions, such as 125mg gel capsules are equally effective. A bunny can safely receive the contents of half a capsule at the rate described above.) SOURCE: http://www.bio.miami.edu/hare/ileus.html  (which is also very helpful to read in it’s entirety)
Many times diet can be the culprit too and normally, if it is diet related, it won’t happen over night but it will happen over time. Also some bunnies tummies are more sensitive than others. What may be fine for one bunny, may not be for another.
What is your bunny’s diet right now? What kind of pellets? What greens/veggies does he get? How much fruit or treats does he get?
Keep us updated about what the vet says.
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All good advice-please keep us posted! 
{{Snugglepuff}}