We cant give a good answer without good information.
What year is the car, what engine, is it diesel or petrol?
Any recent service history?
Any 'tuning' parts fitted, eg cone filter?
Did the problem suddenly appear or did it get worse over time?
Was any work done on the car just before the problem showed? Could someone have left a plug or hose off?
How high are the revs?
Is it worse/better when the engine is hot/cold?
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