My budgie had a similar problem. He was constantly chewing and kept tipping his head back as if he was having difficulty swallowing. He also became quiet and slept more frequently. He didn't talk or chirp as much.
When we took our bird to the vet, she was alarmed to find his weight had dropped to 42g (from 48g about 2 weeks before when he had a general check-up), and said he felt thin. It turned out he had a sore, swollen mouth, some sort of fungal or bacterial infection. This was preventing him from eating properly, resulting in weight-loss. You could see this swelling both above and below his tongue when you opened his beak. We still do not know what had caused it. He had to stay overnight so they could tube feed him and start him on medication. After a series of anti-fungal and antibiotic drops, he seems to have recovered, although we are carefully monitoring his eating.
Have you noticed that Pavarotti is having trouble eating or swallowing? Our bird was unable to properly eat his seeds or his usual favourite celery, but he was very happy when we provided him with millet, which he ate very quickly. According to the vet, being able to eat one thing (eg millet, which is small and easy for them to eat) but not others (his normal seed) is a sign of something wrong with the mouth. It was causing him pain to handle the bigger seeds (and he usually loves his seeds!).
Keep a close eye on Pavarotti, and make sure he's getting enough food in. Check that he isn't getting too thin or losing weight rapidly. If you are worried it is getting worse, or he doesn't seem to be eating well, another visit to the avian vet would probably be a good idea. Budgies can go downhill very quickly, but when put on the right medication will recover pretty fast.
Hope this helps