What is it that makes Terminators noticeable by dogs?

Because they look like humans, but don't smell like them.

If you've ever seen a dog meet a vacuum cleaner, that about sums it up.

As everyone else has pointed out, smell and sound probably the most likely reason.

Dogs have instincts. They also have lower telepathic abilities, which enables them to know when you're serious and when not - they can read people, so you can never lie to a dog. Seriously, even Dog Whisperer often says this - you can NEVER lie to a dog, because dog dan basically 'read your mind'.

Does this answer your question? Dog senses IMMEDIATELY when there's a weird entity in the room, that it has not encountered before. Dogs are very tuned-in to energies, so when there's a sudden machine that has no soul, a dog gets nervous and because it can't read its mind of feel anything from it except purely electric energy and whatever that rotting synth-flesh gives off besides a smell, it instinctively knows something is wrong.

Dogs can also sense 'intention' to an extent, so if you INTEND to harm its handler/owner, the dog knows it and wants to attack you.

However, I don't know if a dog would be able to sense a soulless machine's "program-based" intention to terminate someone - then again, intuitively, it would feel a danger, and that there's something wrong. So yeah, considering all this, it's no wonder they use dogs.

Also, they can be trained to detect and smell these things. Dogs can smell cell phones even through coffee, so why not terminators through synthetic muscle and skin?