Low frequency tinnitus

Hi,

I've had "normal" high pitched tinnitus for several years now, and have adapted/accepted it to the point where I rarely notice it, and can live comfortably with it.
However, recently I've developed a new, far more intrusive low frequency tinnitus that has been plaguing me, and worrying me a lot.

Some days it is barely noticeable, and others it is quite loud and intrusive. It's in both ears, and varies in volume, often pulsating, or apparently resonating and seeming louder when other natural sounds occur, such as traffic passing by. Sometimes it's more constant.
Clapping, or other sudden loud noises can cause it to temporarily dip for a few seconds, then fade back up again.

The frequency of this is around 78hz.
I first noticed the hum several weeks ago, and thought the noise was external as when I put my fingers in my ears the noise cannot be heard. When removing my fingers however, the hum fades back in again over the period of around 10-15 seconds.

I've visited my local GP who has attempted to remedy this by giving me anti-histamines. Great for my hay fever, but has done nothing to resolve the low frequency tinnitus.

I'm petrified that this may be permanent, or get worse - as it's not something that I could permanently live with - it's driving me mad.
I know all the usual things - don't focus on it, distract yourself with music or sound effects etc. but I just want to figure whether or not this is something that may hopefully disappear.

The only other twist to the tale is that I recently had surgery (not near the ears) and it seems to have been worsened since then, though was definitely present before. I also recently had my teeth fully cleaned by the dentist.
Does anyone know if either of these things may temporarily cause this type of tinnitus?

Many thanks in advance for any advice/comments.
I'm keen to see an ENT doctor as soon as possible, who will also hopefully be able to shed some more light on this horrible situation.