These may be hard to track down, here are some of the known causes:
Rear driveshafts, propshaft centre bearing, front brake discs warped and / or incorrect hub adjustment, wheels and tyres, and believe it or not ,engine misfires. An eight cylinder car running with one or two cylinders out can give the impression of a vibration at speed.

Check for abnormal tyre wear, i.e., scuffed edges etc, this may be a sign of incorrect wheel alignment or possibly a worn or damaged steering/suspension component.Steering components are generally not expensive.

Wheel alignment on these cars is a specialist job, basic "tracking" adjustment can do more harm than good and should not be attempted as a "quick fix" A four wheel alignement is the prefered method.

Due to our weather conditions and the amount of salt used on the roads here in the UK SEC`s are prone to rust, so its worth checking the following areas thoroughly:-

Lower edges of the front wings (Mud collects here and rots out the lower corners just above the bumper) Rear wheel arches (in the most part due to leaking rear screens), Chrome bumper trims rust through (especially on the older models) these are expensive to replace.
The boot/trunk panel beneath the rear screen, look inside the boot underneath the rear screen with a torch as rear screens are prone to leaking causing rust, a wet boot area is a dead giveaway. Rear screens de-laminate, they are hard to come by and are expensive to replace.

Tired boot/trunk seals also let water in and are a common cause of exhaust fumes entering the car.

Engine rattles/clatter on start up, then dies away: This can usually mean a replacement timing chain and tensioner is required, this is a relatively inexpensive job and cheap insurance!

A constant clatter or tick: This would suggest a problem with a cam follower or camshaft check for healthy oil pressure at gauge.

As a guide on oil pressure the needle on the gauge should be on the top mark (3) or as near as when cold. However the needle can drop to just over the lower mark (1-2) when hot but this is ok.

Check for puffs of blue smoke whilst driving and on idle, this can be a sign or worn valve seals or guides.Valve guide seals may be replaced in situ without the expensive job of head removal.

Worn gearboxes are prone to slippage or "flare up" between gears under load and is usually a good indication of an impending gearbox rebuild. Visually check the colour of the auto box oil via the dipstick,it should be fairly clean (cherry in colour) and not burnt and black.

Harsh gear changes can usually be remedied by means of adjustment. These problems can also be the product of poor maintenance, i.e. fluid levels to high or low, and / or the possibility of a clogged transmission filter.