Found this on a quick Google search - interesting.
************************************************** ********

Why Does My Cat Snore

Our cat snores so loud, sometimes it wakes us up.

Nevertheless, it's hard to get mad at her when she's sleeping so soundly. Which, turns out, is one of the reasons why cats snore.

Yep, when they are completely relaxed and trust their people, that cat nap can start to sound like a train coming through.

Also if something is irritating the passage or blocked in there, this can hinder the breathing too. Usually a cat sneeze will get rid of the offending irritant, but sometimes if not everything is pushed out, cat snoring can occur due to the blockage.

Other reasons could be:

.Allergies

.Postnasal drip

.Being overweight

.Polyps which are growths that can form in the breathing passageways and block the airway when kitty relaxes and falls asleep.

.Asthma

.Even hairballs.

If while snoring, he is showing signs of excessive breathing problems such as hacking, gagging, wheezing (especially around heavy pollen times) or very overweight, you may want to take her to your family pet health care provider.

In the meantime, while your cat is cuttin' logs, consider it a compliment of high cat order that they trust you so much to sleep so deeply-if not quietly-with you.