Harbor Freight (aka Hazard Fraught) has a set of "use once" 25" long drill bits for $15. I drilled a hole as far as I could get into the center of the mufflers and now it makes a nice burbling sound that isn't loud enough to be obnoxious.
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https://www.harborfreight.com/14-in-716-in-58-in-x-25-in-long-wood-drill-bit-set-3-piece-70927.html
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Tom Herling