Following the Instructions on Whimsical Raps site:
“The particular card has been chosen to be ideal for W/'s purposes. We use a 4GB, Class 10, micro SD card. In testing we tried a lot of different cards, and found this to be the most reliable & fastest for our application. If you’re going to replace the card yourself, we recommend a Class 10 card, and either 4GB or 8GB (note 8GB will not extend your tape time currently). 4GB cards tend to have faster access times. In particular we discourage using the more modern ‘Ultra’ or ‘Extreme’ speed cards- They sound like they’d be great, but W/ uses the SD card in a very non-traditonal way meaning these cards actually perform far
worse than the older ones, and you’ll likely have audio dropouts in your recorded files.”
Maybe it’s just the Hispeed sd to act worst? I don’t know, but I bought a class 10 4gb Kingston sd last week as I had the same problem as you, after a few quick recordings the “sample rate noise” seems to be gone, but I have not stress tested the thing.