I thought Liddy speculated it might have been John Dean.
Liddy has such intense hatred for Dean -- seemingly justified, seeing what a snake Dean is -- that he would probably accuse him of anything.
Nixon and Haldeman knew who it was - so did the whole White House probably.
The following from Slate:
"As in the White House tapes excerpt Chatterbox previously cited, Felt is discussed as a probable leaker. This time, though, Nixon (who, among other things, will likely be remembered as America's last anti-Semitic president) expresses horror at discovering Felt to be Jewish. This conversation, dated Oct. 19, 1972, is even more Gothic than the last:
Nixon: Well, if they've got a leak down at the FBI, why the hell can't Gray tell us what the hell is left? You know what I mean?...
Haldeman: We know what's left, and we know who leaked it.
Nixon: Somebody in the FBI?
Haldeman: Yes, sir. Mark Felt. You can't say anything about this because it will screw up our source and there's a real concern. Mitchell is the only one who knows about this and he feels strongly that we better not do anything because--
Nixon: Do anything? Never.
Haldeman: If we move on him, he'll go out and unload everything. He knows everything that's to be known in the FBI. He has access to absolutely everything ...
Nixon: What would you do with Felt?
Haldeman: Well, I asked Dean ...
Nixon: You know what I'd do with him, the bastard? Well that's all I want to hear about it.
Haldeman: I think he wants to be in the top spot.
Nixon: That's a hell of a way for him to get to the top.
Haldeman: You can figure a lot of--maybe he thought--first of all, he has to figure that if you stay in as president there's a possibility or probability Gray will stay on. If McGovern comes in, then you know Gray's going to be out ...
Nixon: Is he Catholic?
Haldeman: (unintelligible) Jewish.
President Nixon: Christ, put a Jew in there?
Haldeman: Well, that could explain it too. "
I always thought it was John Dean too.