Sounds like a vacuum leak possibly at the carb base gasket. I had a whistle for a couple days, turned out to be the base gasket at the carb. Good luck, don't over tighten those carb studs.
Tom
I pulled a 2100 2v carburetor from our 73 Mustang's 302 engine a few weeks ago. Overall the carb was performing pretty well, although it did have a slight hesitation on light acceleration. The accelerator pump was working properly, and the ignition timing was at spec (6 degrees BTDC). Further, the vacuum ad vance diaphragm was not leaking, and I verified ported vacuum was reaching it when it was supposed to. During disassembly I noted the idle mixture screws were both out a bit further than normal, and the passenger side was out further than the driver side.
I anticipated the float level was likely set too low, which about the only other thing that would cause a hesitation on light acceleration, since the other routine causes were already check out. But, the idle mixture screw was a bit of a mystery, until I thought back to when I pulled the carburetor off the intake manifold. The front passenger nut was not tightened properly from a prior time (before I had the car). The nut was only finger tight. I then looked at the base gasket for the carburetor where it is bolted to the intake manifold, and found the regular "crush pattern" of the carburetor base was not present near the passenger front part of the gasket, whereas everywhere else on the gasket I saw the expected crush pattern. So, I essentially had a little vacuum leak, which is no doubt why the idle mixture screws were turned out more than normal, causing the idle mixture to be richer than normal (unless there was a vacuum leak).
I finished rebuilding the carb, reset the float to its correct level (it was just over 1/4" low, which would have caused the running Air/Fuel ratio to be on the lean side, and would delay when fuel flowing through the venturi tubing from the fuel bowl would be a little delayed - hence the hesitation I was feeling. I suspect someone set the float low like that on purpose to try to get better fuel economy from the engine. It was set so low that just replacing the needle and seat in the carb would not have accounted for that much out of whack it was set at.
Anyway, after reassembling the carb I fired up the engine. The idle was indeed too rich (I turned the idle mixture screws in as far as they had been when I first removed the carb), just like I expected now that the base gasket vacuum leak had been corrected. I also had to increase the idle speed setting at the Throttle Position Solenoid, as the lack of air leak at the base, along with more fuel from the idle mixture screw(s) would have caused the idle speed to be higher than normal. The engine runs great now, no more hesitation at light load, medium load, or heavy acceleration. The engine idle is about as smooth as it was prior to the rebuild, as the idle mixture screws were adjusted in a richer than normal position in an effort to compensate for the vacuum leak at the carb base plate.
Anyway, I never heard any difference in sound, or whistling, when I applied our brakes (power disc), but it is very possible that is what is going on with your system. It is also possible, especially all these yers having gone by, that the brake power booster has a vacuum leak. A diagnostic vacuum pump, with the correct adapters, will help you ascertain if yours is leaking or not. When vacuum is applied to the booster, assuming the vacuum tester has not leaks with its connections, you ought not lose any vacuum applied to the booster. If it is leaking you found the cause of your problem. There are some other tests to perform as well. I will provide some video links below.
There are a lot of different power brake booster for those First Generation Mustangs and Cougars. More than I would have ever guessed. In order to get the correct one, if it is needed, I suggest you call WCCC after watching the following YouTube ink they have published:
They will help get you all set up.
Here are a few YouTube videos showing how to test a booster for vacuum leaks without using a vacuum pump. These are not showing their techniques on an older Mustang, but for the most part the techniques shown work on our First Generation Power Boosters just fine. If you search YouTube you will find a lot of other videos, many of which are effective with our old Mustangs, also:
