That's assuming that the secret is the encryption algorithm itself rather than the key. Modern symmetric encryption does not work that way - the algorithm is public and well known while the key is the actual secret required for encryption/decryption.

I don't see how adding noise in modern encryption can help other than increase the size of the output.

Some modes of operation make use of random noise (IV in CBC, nonce in CTR, etc) because it's a convenient way to get a unique number but it's not for obscurity, it's because it's needed to prevent attacks on these modes.