My 93 240 wagon has a strong gas smell at rear of car. Fuel lines were replaced by mechanic which he said had pinhole leak. Did not fix problem - now wants to replace tank, but no evidence of fuel leak, just fumes. Says it is probably leaking from top of tank, don't want to replace if not necessary, any suggestions. Gas mileage unchanged, car performs well otherwise, 200,000 miles. Notice some buzzing from rear, no ther problem - please advise, thank you