Q: Vibration felt through seat every 5 to 10 secs at idle turning air con on stops vibration

As a car ages, things will not be the same as they were when new. There are vibrations inherent in almost every aspect of a vehicle. The fact that we feel so little when sitting in our cars is an engineering marvel. Diesel vehicles in particular are even more so prone to vibration.

As your car warms up, the idle gets lower and lower. The lower an engine idles, the more vibration will apparent. When you turn your A/C on, your car idles up to compensate for the extra load placed on the motor.

Cars use a number of methods to dampen vibration. Not only engine mounts, but balanced shafts and wheels as well as suspended weights in select locations. As these items age, they begin to collapse, become compressed, and lose their pliability. This is unfortunately a normal part of an aging vehicle. Figuring out what to replace to fix a random vibration can be infuriatingly difficult. Many times a technician can perform some pry bar tests or power braking tests to find what is not performing as new. But this is something that can be very difficult to isolate.

As for the struggles in third gear, I don’t see a connection between the two. Usually if a vibration is occurring, it will occur at certain speeds or RPM’s. Regardless of what gear the car is in. It is possible that third gear is the speed where the frequency is the strongest. Either way, this is likely going to be a difficult thing to track.

Even so, I recommend have a technician take a quick look at it. I say quick, because this kind of thing can take a lot of time with no results. The amount of time spent should be proportional to how bad you want the vibration gone.

If you decide to get this fixed, I recommend getting a certified mechanic, such as one from YourMechanic, who can diagnose the shaking/vibrating in your seat and help you fix it accordingly.

Good luck!