Tbh I find it playable solo or coop. Solo def pays to be stealthy. When it kicks off you'll struggle on your own the ai of your team mates is a bit lame.
Can only echo above. If you can get a gear it doesn't sound like the clutch. When they go they either break up or slip in all gears.
Either way you'll need to drop the box so that would be the first step.
Get it back up to clinic for them to investigate. As said def rotational so could be wheel bearing or similar. Doesn't sound like rear shock nicking either.