Sudden Change in Engine/Exhuast Tone?

I have a 1966 Plymouth Barracuda with a 273 and dual exhausts. The car has dual glass pack mufflers and the whole exhaust system is about 20 years old, so I know it has some leaks. I was out for a drive today and I thought I was going crazy. The engine suddenly changed in tone from its normal. I could only notice it when I accelerated or down-shifted, but not at idle. Idle sounded exactly the same as it normally does. It's hard to explain the change, but it seemed more agitated, and definitely louder, especially if I stepped on it. It ran as normally as it does, no engine skips or sputters. When I got home, I revved the engine and opened up the door and the different sound seemed to be coming more from the rear, by the glass-packs. I think it may be possible a new leak formed near or in one of the glass-packs? I'm currently reluctant to drive it until I figure this out in case something goes wrong. If anyone has an idea about what might have caused it, advice would be appreciated. Thanks.

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