What causes the characteristic chirping sound of old VWs?

You’ve heard it, but you may not have realized that you heard it. When an old type Beetle passed you the car made a distinctive sound that couldn’t be mistaken for anything but a VW. I’ve always wondered what that sound was. Any ideas?

It was a function of the stock muffler (80% and the design of the heads (20%)
VWs with headers and extractor exhausts had very little chirp.

Some of the hummingbirds around here make a similar sound. I can’t hear one without thinking of the other.

I think it’s from the flat engine design. A neighbor has an older Subaru and it sounds just like an old VW, it also has a flat 4 engine.

Not so sure about that theory. I had a Citroen GS 1220 with an air-coled boxer engine and it sounded nothing at all like a beetle.
Up untill 1995 Alfa-Romeo’s had boxer engines in some of their crs, none of which sounded beetlish.
On the other hand, I have a '94 Accord that sounds like a beetle under certain conditions, following a rear-end shunt.
There is no discerible damage to the exhaust sysem although the car is a few inches shorter now than it was.

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On the old “flat four” air-cooled VW engines, the cylinders (jugs) were separate from the crankcase and the cylinder head. Long studs threaded into the case and the cylinders and heads slid down on these and where held down by nuts on the top of the studs.

It was common for the studs to stretch over time and loose some of their tension. Occasionally the studs would even pull out of the crankcase, but this usually only happened on engines that had been modified for more power. A fairly common repair when rebuilding these engines was drilling out and Heli-Coiling the crankcase threads to a larger size to keep this from happening. It was almost manditory if you had installed larger cylinders or added a turbo/supercharger to the engine.

When the studs stretched and lost tension, the heads would slightly raise off the cylinders and the cylinders would raise slightly off the crankcase. It was air and combustion force leaking through these areas under acceleration that caused the infamous VW whistle as it went by. When the cars were new, or after repairs had been done in this area of the engine, the cars would not make this sound. It was something they developed over time.

I know the sound you are speaking of, and the chirp that I am thinking of is not the same noise.
Brand new engines with stock exhausts would make the chirp. Old engines with weak studs, or bad valves made a different sound.
But it has been 35 year since I was near a windsucker with a wrench.

Former Beetle owner who ran several exhaust systems-only the stock was chirpy.