Here is a funny thing that happened to my car last week:
It was time for emissions inspection, so (among other things) I decided
to check the valve clearance adjustments. While doing so, I
found a broken intake valve spring (the big outer one). The funny thing
was that I had not realized this while driving the car. I had not
noticed anything unusual in how the engine ran. The last time
I had the valve cover off was about a year ago, and I have driven
the car maybe 1000 miles since then.
After replacing the spring (did you know it is possible to replace the #3 valve
spring without removing the rocker arm?) the engine seemed to have
just a little bit more midrange and high-end oomph...