Trailer vibration while loaded

[HDRAM2020]

I just bought a 18ft Aluminum tilt deck trailer with 3500lb torsion axles and when I am loaded at different speeds (mostly higher 75 seems the worst but does it at 70 some) I get a vibration, I first thought tires on trailer and just put new Goodyear Endurance radials on (it had bias tires which looked great no uneven wear at all). Empty you never feel it. And different loads form hauling my SxS ,mower, and Kubota tractor all have it even when hauling a heavy load with mower and SxS on it. Doesn't make sense. I have pulled my small trailer 12ft with SxS on it never a issue and pulling my Gooseneck 25ft equipment trailer and my 28ft boat, nothing. So leads me to this trailer . Hopefully not truck. but in all my years pulling never had a issue unless a bad tire and you can see it bounce. Bought trailer used it's a 2018 High Country. Any ideas would be helpful. My truck is a 2020 3500 SRW Mega cab Limited

 

[GBB]

Your Trailer tires may need to be balanced. I always balance my trailer tires..equipment trailer, boat trailer, travel trailer, etc.

 

[H3LZSN1P3R]

Is it sitting level when loaded if it is on to much of an angle you can have axle hop...

 

[HDRAM2020]

Tires were balanced when installed last week, Have changed height of trailer at hitch, might try going higher. not any sag in truck when loaded with a 3500 and air ride. I have never had a aluminum trailer wondering if it's flexing a lot .

 

[H3LZSN1P3R]

Tires were balanced when installed last week, Have changed height of trailer at hitch, might try going higher. not any sag in truck when loaded with a 3500 and air ride. I have never had a aluminum trailer wondering if it's flexing a lot .

Lol i missed the aluminum part.... thats the issue they are junk to much flex for a hd trailer i would unload it and buy a steel galvanized replacement

 

[HDRAM2020]

Maybe so but I hate to give up yet. I know they can crack and have issues have had to repair some before for other people but they were made pretty crappy. This one is well built with a lot of extra gussets and basically 2 frames being a tilt trailer and it pulls great when empty or lighter load no bounce I can see, thats one thing I was worried about because of trailer weight being 1400lbs.