So I was doing the final quest for the Companions. I took a carriage to Winterhold and continued to my destination on foot. There was a small river in front of me and decided to swim over it. I started swimming and just before I reached land I heard a very strange noise. At first I thought it was a Dragon but then I realised that I had not started the Main Quest yet. I am confident that it was not a Wolf, Troll or a Bear. The sound reminded me of a Dragon but it was a lot deeper and I didnt hear it again anymore. I entered Sneak mode and it showed that I was being watched but I didnt see any red dots on my compass. Has anyone had an experience such as this? Any suggestions on what made that noise?

Horkers make a very deep rumble (lots of bass) when you're fairly close to them...
But that's spooky, eh?
I agree; I HATE when the eye says something is watching me, but I don't know what it is.

An invisible snow whale..

maybe it is a sprigan they are invisible

Could have been a Horker or the glaciers.

Ive never known spriggans to reside in that area. More than likely it was a horker.

It might have been a giant horker rolling over.

I've seen Spriggans in a random encounter in the trail between Winterhold and Saarthal.

My apologies, my stomach tends to rumble like a rock band when it gets empty.

Hevnoraaks gaint dead horker.

Near Azura's shrine, their's a carriage with dead refugees. When you get close, you'll hear some kind of roar and some rocks will fall. I guess that was made to indicate there's a dragon nest nearby, even though it's occupied by mages until you start the MQ.

Beware, ancient aliens are real

or it could be a landshark.

T'was a yetee !