90k and auto transmission. Sound comes from the right front - specifically at or under right side fire wall. Sounds like a train horn at a distance or lifting metal lid of large dumpster. Happens about 3 out of every 4 driving events at no particular time. Happens only once during a driving event (key on/key off). Always at 15 to 25 mph. Lasts 3 to 4 seconds. Same smooth uniform frequency from start to end. On a few occasions, the sound is immediately followed by a much quieter version lasting about 2 seconds. Not related to braking, accelerating, putting in gear, shift points, reverse or turning. Person in right front passenger seat with feet on slanted part of floorboard will feel strong vibration in right foot. No other odd car behavior when sound occurs. Started 6 weeks ago. No recent repair, maintenance or damage events to consider as cause. Just got back from 1,000 mile trip and it ran like a top. Felt front wheels every 200m and they stayed cool. But the odd sound occurred nearly every time I drove it. Car is serviced and inspected every 2,500 miles. New transmission and timing belt one year ago. Brakes and tires have less than 10k. Both in good shape at last service.
This is a first for me. I don’t have a mechanic’s ability or knowledge, but I’ve owned 16 vehicles over 50 years. Help please.