Off and on vibration every 5 seconds

Going to have to narrow it down a bit.
I’d say check driveline, carrier bearings, U joints to start.

 

First thing to check would be wheel balance and maybe a flat spot on one of the tires. Was it doing that when you test drove it?

 

Check the carrier bearing. Or replace it anyway, I chased a vibration in a 2017 Pete 579 for months and finally replaced the carrier and it pretty much all went away. Mine would do it around 65 to 70 and then stop until about 85. On throttle or off the throttle didn't matter.

 

I would start with the tires. Do they have an uneven wear pattern, flat spots ect. Then check the drive line, u-joints, carrier bearing.

 

Have a couple of those here. Different drivers and different tires. Still trying to figure that out myself.
They were all transfered here with some mileage on them. Wondering if the driveshafts yokes should be off set as someone else pointed out in an other post.
The yokes on those are longer than standard shafts. We had to replace an inter axle shaft on one of those. Went to the local driveshaft shop. Fit in and looked fine. A couple of weeks later had a worse than usual vibration compliant. While driving over very uneven ground the short yokes pushed against the strap bolt heads and spread the yokes. Bought through shaft and rear pinion yokes, plus driveshaft from Mack. Problem solved but the driveshaft shop talked in circles. I set them pictures.

 

I just chased down a vibe and made a tread about it. It was a vibe that only happened in 7th dir or od at 40-50 mph. Turned out to be an ever so slightly loose trans output shaft yoke nut. Could not see any movement in it when shaken by hand. Only thing that gave it away was a little oil leak started from between the nut, washer and yoke. Seal did not leak.
Doesn’t sound like the same vibe but worth a check anyway.

 

I have a similar vibration. It starts strong about 45 thru 55. Speeding up, steady speed in the range or slowing down. Faint at other times. I think depends on the road surface.
Tires OK no odd wear patterns on truck or trailer.

The cause I found today.
The Nut on the drive shaft that the U-joint is near the carrier bearing.
Doing LOF today found it with flashlight. Usual inspection without the extra light, I couldn't see the faint rust trail or see the visible, very slight movement, behind nut on shaft.

 

A vibration like you describe can be a wheel end. Brake drum out of round, tire not on rim evenly, but most likely sounds like a loose wheel bearing.