Going by my gut, it doesn't sound as if it's speed related.
60 km/hr is less then 40mph and it's too low a speed for wheel balance issue, etc.
To check for possible vibes coming from a loose part, related to RPM, I'd try first to lock the tranny in first gear and drive it to same RPM that you reach normally at 60 km/hr and then repeat at 2nd gear.
Then I'd try same RPM range without driving at all.
Any loose screw/washer or any other parts can cause this kind of humming. Try to evaluate the pitch and frequency and try to match it to a possible small/large loose part or solid vs sheet metal, etc.
Also, broken/loose pieces in a cat (if you have one) may manifest itself in some cases with humming vibes.
Good luck!
Jay
60 km/hr is less then 40mph and it's too low a speed for wheel balance issue, etc.
To check for possible vibes coming from a loose part, related to RPM, I'd try first to lock the tranny in first gear and drive it to same RPM that you reach normally at 60 km/hr and then repeat at 2nd gear.
Then I'd try same RPM range without driving at all.
Any loose screw/washer or any other parts can cause this kind of humming. Try to evaluate the pitch and frequency and try to match it to a possible small/large loose part or solid vs sheet metal, etc.
Also, broken/loose pieces in a cat (if you have one) may manifest itself in some cases with humming vibes.
Good luck!
Jay