I've been wanting to do this for some time. I hope that 10 seconds of a wav sound file isn't too long. I'll pare it down if necessary. I'm on a phone line myself.

First, here's a picture of my 261 taken last spring.
My 261

Here's it starting (6 volts!). It runs so quitely that you can only hear fan and belt noise. The microphone was near the distributor.
261 Starts and Runs

I put the microphone down the breather tube. Don't tell me that you hear something, like that doink-doink, that says it's going to blow up. I want to be surprised!
261 Sounds from the Breather Tube

Here's the sound from inside the valve cover.
261 Sounds from Inside the Valve Cover

For you engineers: How about a Fourier transform to determine the individual frequencies?

1948 3/4-Ton 5-Window Flatbed Chevrolet

34 Years. Now with a '61 261, 848 head, Rochester Monojet carb, SM420 4-speed, 4.10 rear, dual reservoir MC, Bendix up front, 235/85R16 tires, 12-volt w/alternator, electric wipers and a modern radio in the glove box.