00Silver

My grand marquis makes an odd "thump" which seems to be coming from underneath the passenger seat somewhere under the car. It only does this right after you start the car, but it does it every time you start it. It sounds as though someone is laying underneath and slapping the bottom of the car. Sometimes its a hard "thump" and other times its barely noticeable, but its there every time. Usually about 5 seconds after the car is started is when it happens. Any thoughts or ideas? Thank you

R.S.LOGAN

Checked engine mountings / exhaust mountings ? though would have thought it would have made noise on start up , rather than 5secs later .

00Silver

Exhaust is tight, as are the motor mounts. Those were my first thoughts as well. Possibly cat. converter on its way out? The cat. on that side has a rattle inside of it. I know it needs changed but could it cause my "thump"?

R.S.LOGAN

If the cat is rattling / breaking up inside you can get all sorts of problems with blockages , build up of pressure etc. Might be your answer .

00Silver

Anyone know where to get a decent direct fit cat. for cheap? Autozone has em for $250+ and I've found them on ebay for around $200. I'm currently laid off and have a 3 month old daughter so money is very tight at the moment. Would it be worth finding a used cat. from a junk marquis or should I definitely buy new? I appreciate the replies

R.S.LOGAN

Really better with a new one , but with your present circumstances , if you can get a good used one cheap enough , might tide you over .All depends on how much they are asking and whether the difference in price is worth it . All the best .

00Silver

Thanks for the quick responses and advice