During my last course called "Physical Examination of the Abdomen" (at medical school), we learned to palpate the abdominal aorta, which can be easily palpated in +/- lean patients.
This book (freely available here http://www.ncbi.nlm.nih.gov/books/NBK350/)
"Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition" gives you a description on how to palpate the abdominal aorta and listen to possible bruits:
The abdominal aorta is an upper abdominal, retroperitoneal structure
which is best palpated by applying firm pressure with the flattened
fingers of both hands to indent the epigastrium toward the vertebral
column. For this examination, it is essential that the subject's
abdominal muscles be completely relaxed; such relaxation can be
encouraged by having the subject flex the hips and by providing a
pillow to support the head.
Here a figure from the book:
The abdominal aorta is retroperitonal so it is possible that some bowel distension or feces impaction (which can occur in IBS) might influence the palpation but there is no direct correlation between IBS and the impossibility to palpate the abdominal aorta. Factors that for sure influence the palpation are obesity and massive abdominal musculature.
Finally (in contrast to what has been suggested in a previous answer), I have never heard about "palpating the celiac artery" and I don't think it is possible to really palpate (and be precise enough to say it is the celiac artery) it through all the organs which surround it (see image below, some parts of the liver have been removed to be able to see the artery)
Sources: Figure 2: wikipedia: https://en.wikipedia.org/wiki/Celiac_artery