>>size db ?
>>size ram ?
>
>The database would be around 200 GB.
>
>The ram is 38 GB.

To date, my experience with SQL Server,
tells me that if the relationship SIZERAM / SIZEDB is very small 1
it is common to have performance degradation.
Certainly the size of RAM it must be larger than the size of the Client table including the indexes.

Another issues it is a bad optimization of @variables.

Do this test:
restart SQL service
exec this:

DECLARE @NoEntity Int
DECLARE @VehicleMake Varchar(30)
DECLARE @VehicleModel Varchar(30)

SET @NoEntity=19
SET @VehicleMake='VOLKSWAGEN%'
SET @VehicleModel='"CABRIOLET*"'

SELECT Client.PrimaryKey
 FROM Client (NOLOCK)
 WHERE Client.NoEntity=@NoEntity AND Client.VehicleMake LIKE @VehicleMake AND Contains (Client.VehicleModel, @VehicleModel)

-- time 1 ?

SELECT Client.PrimaryKey
 FROM Client (NOLOCK)
 WHERE Client.NoEntity=19 AND Client.VehicleMake LIKE 'VOLKSWAGEN%' AND Contains (Client.VehicleModel, '"CABRIOLET*"')

-- time 2 ?

post time 1 and time 2.