Exactly that.

I have a couple of clans that seem to hate me for absolutely no reason, raiding me constantly.

At first I counter-raided and all was well, but I later turned to a more diplomatic way. I've gained the trust of most clans, but those two won't let it be.

I am NOT feuding with them, and I've supported them in encounters that have something to do with them. I've send them gifts until they've apparently turned friendly, only to have them raiding me again. This is really frustrating, but I'm sure there is something I'm doing wrong that seemingly makes them hate me.

My current suspects are longstanding grudges (I did clash with them early on in my existence), but I don't know how to appease them. It is apparent that I cannot fight them untile they want peace, since my diplomatic pursuits have left my army in disrepair, and it would take several years to get them in suitable shape.

This is really starting to hurt my economy, if someone has good ideas, I'd appreciate!

(at least I'll know to avoid widespread warfare on a balanced clan from now on ; D)

There are numerous dimensions of your relationship with other clans, which can indeed include such things as having raided them, or various slights. And these are in fact difficult to erase. Gifts can improve other dimensions, and that can in turn reduce raid likelihood.

Thanks for the answer!

I think they calmed down quíte slowly, but they did calm down eventually. I've got new archnemeses already. : D

I also think this clan is ruined. I lost my Earthshaker cavalry and a ring member in a botched heroquest, and the carls are packing up... I suppose it was a good run! Fun, in the very least. A fantastic game.

Also note that a raid doesn't always mean "they hate me". Raiding (especially cattle raiding) is a form of sport for these people, so they will sometime attack a random neighbour just because they want a few cows or because the warriors are bored.
It's when you are constantly raided by the same clan and the bodycount rises (and the objectives switch from "looting" to killing you or burning your home) that it becomes personal :)

Sometimes you get lucky and have your arch-rival ask you for peace. It happened to me in my playthrough - I had been agitating another clan (I guess other clans are also Warlike, Balanced or Peaceful?) as a Peace clan and each year I would suffer incessant raids from them. Occasionally I would retaliate, and the spiral of violence would grow deeper. To my surprise, they later sued for peace not once, but twice, since I botched the first parley. I think this actually happened with two different clans - the last such suing for peace was especially well timed, as I was about to start down the road to form a tribe (or possibly become the monarch of the Pass, I forget which) and was pressed on resources enough already.

I'm not sure, but I think a combination of maintaining a good image and dragonewt raids on their lands might have been what helped end the feud. Of course, the Orlandi are fickle at the best of times, too.