I'm sure you've come across these over the years. The Beatles had many on their recordings.

My question is what is the correct Technical name for making these (And its not Subliminal messages as most people will say)
1 word answer

Google led me to Wikipedia: "Backmasking is an audio technique in which sounds are recorded backwards onto a track that is meant to be played forwards. Backmasking is a deliberate process, whereas a backward message may be unintentional. Backmasking has been a source of much controversy, especially related to supposed subliminal messages in rock music."

Thought that would be harder? There are quite a few sites with clips of these also text lists (beware the language is sometimes very coarse) This is the biggest collection
www.backmaskonline.com/mp3.html