Rear differential whine at highway speed — 2008 Toyota Tacoma

The noise pattern tells us a lot: - **Whine on acceleration only** = drive-side gear wear (pinion/ring gear) - **Whine on deceleration only** = coast-side wear - **Both** = bearing wear Yours (acceleration only) points to ring and pinion gear wear, likely from running on old fluid for 142k miles. The diff fluid breaks down and loses its extreme-pressure additives, which are critical for the hypoid gears. **Before you panic:** 1. Drain the diff fluid and check for metal shavings (some fine grey paste is normal, chunks are bad) 2. Refill with Toyota genuine diff fluid or a quality GL-5 75W-90 3. Add 2oz of friction modifier (required for limited-slip, won't hurt open diffs) 4. Drive 500 miles and reassess Sometimes fresh fluid quiets things down significantly. If it doesn't, you're looking at a ring & pinion job ($800-1,500 depending on shop).

My '06 Tacoma did the same at 130k. Changed the diff fluid (it was BLACK and smelled burnt) and the whine dropped by about 80%. Still there faintly at 70mph but totally livable. Been another 40k miles since with no change. Don't skip the diff fluid change again though — every 30k is the schedule.