I recently replaced a wax ring on an old top tank toilet. The bowl sits about four feet lower than the upper tank, After running around town for hours I realized that the specific gasket that I needed was not being made (or not available). I made my own to connect the flush pipe from the tank to the bowl.

No leaks, flushes strong, great. Now it makes a loud groaning noise when it is flushing. Is this the gasket? A lot of parts got jostled and I would be surprised if I broke something this entire toilet was built before gravity according to the local shops.

Could be caused by a number of things. Fill valve may be damaged internally or have debris, restriction in flush valve or supply valve washer (since you turned it off, then back on the washer may have come loose). Does the sound stop when the toilet finishes filling the tank? You can start with replacing the shut off valve and see if that helps then work your way towards the fill valve. BTW, the parts that you needed are still available through most plumbing supply houses, the guy behind the counter just needs to do his job and order them for you. Gravity is the primary power source of almost all toilets in use today.