Whining Engine Noise at Takeoff

Air bubbles in a pump is called “cavitation” which cause less fuel transfer and that funny noise because the pump is trying to pump air (more noise) vs all fluid (quieter).

Much of Arizona allows for a slightly higher vapor pressure for auto fuels during the summer months.  Where many states require summer fuel to have an RVP of 7 PSI (similar to Avgas), much or Arizona allows fuels to have an RVP of 9 PSI.  Avgas is always 6-7 PSI.  Also, if this happened when the aircraft still had some of the last winter grade fuel, the RVP of that fuel may have been 11-13 PSI, which would explain this.  

--UPDATE--

I've been using pure AVGAS yet the noise returned several months ago.  This time I did some more dedicated troubleshooting and found the following:

Chris

Thanks for the update. Just to put the fuel pump thing to bed, does the sound change with one or both pumps running?  

It's the same noise regardless.  I tried isolating each one, then both and it made zero difference.  Frustrating!  😕