There is a known link between PG (propylene glycol) and tinnitus (ringing in the ears). This was discovered, not from vaping, but from ear drops that contain PG:
http://www.dizziness-and-balance.com/disorders/bilat/ototoxic_drops.htm
When you vape, vapor spreads throughout your throat and sinus cavities, which are connected (via the eustacian tube) to your middle ear. PG is known to be ototoxic.
I vaped using PG for months before I noticed my tinnitus worsening. I've had tinnitus since I was a kid, unrelated to ear drops or vaping. A little google-fu led me to many articles showing a possible connection. I switched from high PG to low PG liquids. No improvement. I switched to 100% VG base, and the tinnitus quit worsening, but it was still far LOUDER than before I began vaping.
I already DIY'ed my own liquids. A few months ago, began limiting myself to 100% VG base and no more than 3-5% flavoring, if the the flavoring is PG-based. It took 8 weeks to notice any improvement, but it has continued improving since then. If I vape a little too much PG throughout the day, the ringing gets louder. Even vaping plain, unflavored VG - it takes a couple days to recover from it.
If you've noticed new or worsening tinnitus since you began vaping, PG may be to blame. Google propylene glycol, ototoxicity, and tinnitus to do some research, and consider using maximum VG liquid instead. I've talked with many other vapers who have had issues, including louder ringing, changes in tone, and throbbing sounds. Some never had tinnitus before vaping, but the majority did. Several I know who have drastically reduced PG in their liquids have noticed improvement. Some noticed quickly, most took many weeks or months. Some had only to drop the VG in their nic base. Some had to move to zero-PG, even using alcohol-based flavorings. I consider myself fortunate that I can apparently tolerate some PG-based flavorings, because there aren't many alcohol-based flavorings I like.