I'd say your options are:

- Caffeine (my personal favorite), ingest to the point of random muscle twitching. Available via coffee, pills, gum, and energy drinks, and instant coffee crystals - just a pinch between your cheek & gums). Some people like to combine caffeine with nicotine for the additional vasoconstrictive/tachycardic effect but I'm a caffeine purist, I'm anxiously waiting for the day the FDA approves injectable caffeine for over-the-counter sale.

Since you may have concerns about the effect of caffeine on your health, you may be limited to:

- Plan your trips to remain in a 40 min radius of your home.
- Stop every 30-40 minutes to catch a cat nap or get out and walk around.
- Instead of drifting off to sleep, induce a state of self-hypnosis and "recondition" yourself to feel refreshed.
- Get somebody else to drive you.

If your engine noise is causing drowsiness try drowning it out with music that you find obnoxious (probably available free of charge somewhere on your FM dial).

In all seriousness, if this really is a constant occurrance, I'd strongly consider Matt26's thoughts about an exhaust leak.