add noise to the cleartext to obfuscate the true text among a bunch
I need an attacker to believe that the message is not encrypted
So first of all, a small disclaimer. You realize that regular encryption standards would be much stronger and existing libraries are designed to handle data. On top of that it's generally not a good idea to write your own crypto. This is an irregular request, but what you're asking is still understandable since not exposing the act of encryption has it's own advantages in terms of security.
Now, if you want the data to look authentic, you could go with a PRNG to XOR (link) the binary form of the data. This is actually related to the one-time pad, which is considered strong if used once. You could also use FPE (link) where you basically set the format to some specific length and use that as a block and permute the integers within it.
You could also use some kind of cipher and then put an anagram (link) together so that the original word(s) get obfuscated. This probably won't be expected because it takes more computing power to find a sensible anagram (may need to check a dictionary), and then provide some key to allow decoding. Also, it's not considered very strong because a supercomputer would just rip it apart.
Realistically, if you're deadset of making this, I'd use some form of strong encryption (like AES for any symmetric or RSA/elliptic curves for asymmetric) along with it maybe as a precursor, then try to piece the resulting blocks back together in some kind of believable format that looks like a combination of numbers/letters that look like a real message. The amount of programming involved in doing that, and the computing power needed to run that kind of code, is why you probably won't see this being done very often. Most systems just encrypt and that's considered plenty secure.