Noisy toilet cisterns are a source of annoyance for many homeowners. Hearing the flow of water into a tank isn't a problem, but when that sound is accompanied by a whistle, howl, moan or scream, it can be a little aggravating. Before you replace the whole toilet, check the fill valve, for you might be able to quickly fix the problem.
Fill Valve Issues
Water from the main flows through the fill valve into the toilet cistern after each flush to replenish it. If, during this process, the cistern emits a loud noise, it may be that the fill valve is partially clogged. Clean out the fill valve and see if the noise doesn't stop.