PA-28 Flight Instructor reported a reduction in RPM's while on downwind and was unable to advise ATC due to frequency congestion.

Narrative

I was established on the downwind (talking with tower) and was flying and was going to do the landing when I noticed once established in the downwind for XXR that I started to have a reduction in RPMs. I turned on carb heat since it is a part of the before landing checklist when established in downwind which did improve my RPM's for about 30 seconds. Then my RPMs started decreasing again. I was unable to make a PAN-PAN call since tower was busy moving aircraft holding short of XXR across so then they could transfer over to ground. By the time he stopped clearing people across and making calls he cleared me to land. It was getting to the point I was going to turn in and step on him since I did not want to get outside of glide range. I had remained high enough that I could have glided to the runway if my engine did die but it is still a major safety issue that I was unable to make any sort of call in the first place. I am not at all upset with tower or blame them since they are doing what the FAA has told them what to do. The FAA needs to make changes within its own ATC regulations that allow for pilots to make emergency calls without fear of not being heard. Tower and ground need to be separated even for runway crossings at ZZZ since it is so busy. I am not the only person within the past year that needed to make a PAN-PAN or MAYDAY call because tower was busy redirecting traffic due to midair or ground collision concerns.